The Earth is dong two types of movement in space - it's rotating on its axis, and it's revolving around the sun. Rotating on its axis gives us day and night - because sometimes we're facing the sun, and sometimes we're not. Revolving around the sun gives us our seasons, because of the Earth's tilt relative to the axis of the sun.

But here's the thing - those two movements don't quite match up. It would be really cool if they did, but they're just a little off. So today is November 30th, and it would be awesome if next year on this day (Nov 30, 2023), the Earth was in the exact same place relative to the sun. But it's not - it's about a quarter of a day behind. It's not a big difference, but over the years, that difference will add up. We're used to the winter solstice being around December 21, for example, but if we didn't adjust our calendar, it would start creeping up sooner and sooner. Which isn't a bad thing, but we're creatures of habit, and it's nice to have these things be more consistent.

So for consistency, we just mess with our calendar so that the year's movement and the day's rotations keep on track with each other. That maths out to roughly one day added every 4 years to keep us on track - we also skip leap years every century (except every 4th century) to keep that on track even closer to the true value. It's just a necessity because our day measurements and our year measurements don't exactly correspond with each other as neatly as we'd like.