>Can someone explain what spacetime is?

Honestly? No. No one can. We can model it very accurately, but we don't really know what it is.

>Also does "one dimension of time" imply that there are multiple dimensions of time?

An easy explanation is, imagine I ask you "Meet me in the conference room on the 3rd floor of the building at 1st street and Main Street."

That room has three coordinates in space, X Y and Z, that you must traverse from your current position to arrive at. However, if we want to meet there, you also need a time coordinate. We also traverse from our current time to subsequent times, or at least, that's what we experience. Thus, we'd arrange our meeting with four coordinates, X Y Z and t.

Time is just how we measure changes in the spacial dimensions we observe. No time? No change. Hence, Spacetime. Without time, space would never have changed state to begin with, and everything would still be at a singular infinitely small point without space, as we theorize a pre-big bang universe would be. At that point all I can imagine is quantum indeterminacy spontaneously manifesting a universe from nothing.

In my opinion (assuming the variables eventually add up) the net total energy of our universe is zero. What that really means is the cosmos and all the spacetime therein, is zero energy. From the outside looking in (if there were such a thing) there's absolutely nothing here. There isn't even a here at all. It's something the photons emitted at the beginning of our universe that will get absorbed at the end of our universe experience. Time doesn't pass for a photon, so from its perspective, nothing is here, nothing has happened, is happening, or ever will happen. The beginning and the end are the same instant.

Maybe I went a bit too existential with this question, but the honest answer is we have not the faintest idea what spacetime actually is. Functionally, it's just the name we've given the model we use. I'm good with that. It's a solid model, that only breaks down under such extreme mass conditions, you'd need to observe the inside of a super massive black hole to figure out a better model.