If you were asked to describe how you can move through the Universe, you’d probably think of all the different directions you were free to move in. You could go left or right, forwards or backwards, and upwards or downwards; that’s it. Those three independent directions, described by something as simple as a grid, describe all the possible ways once can move through space.
But those three dimensions are far from all there are. There’s a fourth dimension that’s just as important, even though it’s very different: time. We’re always moving forward through time, sure, but it’s just as much a dimension as any of the spatial ones.
Whether you say we live in a four-dimensional Universe described by the fabric of spacetime, or a 3+1 dimensional Universe, where we have three spatial plus one time dimension, you cannot separate these entities from one another while still being physically correct. Let’s try and understand why.
Human beings, for the most part, live only on the surface of the Earth. When we want to describe where we’re located, we typically only have to give two coordinates: a latitude and longitude. We only need these two values, which describe where we’re located along the north-south and east-west axes of our planet, because the third dimension is a given: we’re on the Earth’s surface.
But if you’re willing to go either underground or into the air above Earth’s surface, you’d need a third coordinate to accurately describe your location: altitude/depth, or where you are on the up-down axis. After all, someone located at the exact same latitude and longitude as you — the same two-dimensional coordinates — could easily be in a subterranean tunnel or an overhead helicopter.
They aren’t necessarily at the same exact location; you need three independent pieces of information to pinpoint your location in space.