When we think of dimensions, we want them to have a few properties:

• they are continuous

• objects can have different locations in them

• objects can interact only by having a small distance between them

The normal 3 spatial dimensions satisfy all of these. Time does as well - a ball 1 year from now cannot interact with a ball right now, but two balls in the same time (and in the same space) can bounce off each other.

These are nice, but they don't particularly matter, because the spatial dimensions have an extra important property - you could rotate some object or path through them, while time acted as a kind of separate background dimension. They seemed to form two separate structures - you could move around through space, and you moved through time at a constant rate.

Einstein discovered that wasn't quite right, and actually, the exact shape of a trajectory through spacetime could be tilted, with the amount of the trajectory that looked spacelike and the amount that looked timelike interchanging. For example, a clock could be viewed as standing still and moving 1 second through time, or moving very quickly and moving 2 seconds through time, depending on how you are moving relative to the clock. It's akin to one person saying that a car moved left, while another person says that it actually moved up and to the left, because they're looking at the car from different angles. This means that there is no way to go back to the view that time is just some background entity that has similar properties to spatial dimensions - time is something like a spatial dimension, with time intervals and spatial intervals having a shared underlying nature (though it's important to note that they are still different, since time intervals act like a negative distance, and that's connected to the reason we cannot move backwards through time).