It doesn’t mean the energy itself is renewable, only that the sources aren’t spent when using them to make energy. The sun doesn’t burn out faster when we use solar power, the wind won’t stop blowing when we use wind turbines, and the rain won’t stop falling when we use hydroelectric or tidal power. This is contrasted with fossil fuels like oil, coal, and natural gas, which get burnt up to make energy and cannot be reused.

It isn't renewable as in "it can be used again". The term renewable is to distinguish it from non-renewable energy which is energy from fossil fuel which the earth has a limited supply of.

Most renewable energy is sourced directly or indirectly from the sun which is, for all practical human purposes, an unlimited source of energy.

Direct solar is energy captured through solar panels. Indirect sources are like wind (caused by the sun heating the atmosphere), hydropower (sun evaporating water to cause rain that forms rivers that can be dammed) or biomass (plants that grow because of sunlight and used as fuel).

"renewable" energy not from the sun would be something like geothermal energy. It isn't strictly renewable but there is a lot of it, if we can tap it.

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Basically if the “source” of the energy is replenishable and not spent. For example, wind is renewable as it replenishes itself and oil/gas is not because you burn it up to produce heat/energy.

Basically it involves exploiting natural processes to "renew" the energy for us.

For example, you could have a large, elevated weight as an energy source. When it falls, it produces energy. Problem is, if you want to use it again, you have to get that weight up high. If you do it yourself, you are just going to expend more energy than you get out of the system in the first place.

But let's say a giant bird flies by and picks the weight up for you. In the end, more energy was still put into the system than you get out of it, but the bird paid for that cost, not you, so you don't care.

This is basically how renewable energy works, but for natural processes driven by the world or sun.

The Earth spins and the sun unevenly heats it, creating wind. We can use wind to spin turbines. The Earth and Sun are paying the cost to spin those turbines, but we are reaping the rewards.

Same for hydroelectric. Water falls and turns a turbine. The sun heats the water, evaporates it where it turns into clouds, the rains back down again, refilling the reservoir we use to turn the turbine.

So any time there is some natural process that operates on short time spans, and we can use that process to get energy out of it, that's renewable energy.

“Renewables” aren’t really reusing energy so much as the source of the energy can or will be replenished rather than running out on human timescales.

For instance, we can burn oil and we can burn trees. There is currently a finite amount of oil on the Earth and a finite number of trees.

But we can grow more trees. We can’t make more oil. So trees are renewable. Oil is not.

The overwhelming majority of “renewable” energy sources that will not run out are actually just capturing energy put out by the sun.

If we want to be very technical, that energy is also finite and will run out, but the sun will expand and destroy the Earth long before that happens, so worrying about running out of that source of energy doesn’t seem particularly worthwhile.

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When you extract and burn oil, that oil no longer exists. Eventually, our reserves of oil will be gone. However, no matter how much wind a turbine captures or sunlight a solar panel collects and turns into electricity, that has no baring on how much wind and sun will be available tomorrow. Nothing has been depleted, as there is an endless supply of wind, sunlight, etc.

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Renewable might not be the right word really. Think of it in terms of when an energy source will run out. Take a disposable battery. You get an hour or two of running something small, and then it's empty. Plus it took work on our part to put the energy into the battery. Now think about the sun. Technically it will run out of energy in billions of years, but for our life spans there's an infinite supply of energy, and we don't really have to do anything to cause the sun to provide energy (aside from devising ways to collect it). So renewable energy would be a source of energy that is going to keep producing without us doing anything to make that source produce. We don't have to do anything to make the wind blow, or waves move, or the insides of the Earth hot. So if we collect energy from those sources it will renew itself.

When we talk about renewable resources we are talking about resources that we can use, and then after waiting a bit they are still there. They will renew themselves. If we dig up a lump of coal and burn it, then it is not going to somehow renew itself and become new coal. But for example if we chop down a tree from the forest and burn that, then over time a new tree will grow in its place. We just need to make sure not to chop down the entire forest and destroy the ecosystems that support the tree. And no matter how many wind turbines we put up the wind is still going to blow, no matter how much water goes through a hydro power plant there is still more rain filling up the rivers, no matter how many solar panels we build the sun is still going to shine. All of these are energy from renewable resources and can in theory be used indefinitely.

Nuclear is often thrown in with the renewable energy sources even though it is technically not renewable. It is just going to take extremely long time before we have used up all of our uranium resources, and probably thorium as well. When we finally get a commercial fusion power plant it will have even more available resources but still technically not renewable.