We live in a four dimensional world.
There are the usual 3 dimensions of space, x, y and z as we say it.
There is a fourth dimension called time.
Einstein's theory of relativity merges the space and time as a 4D world called spacetime.
When we say that a body is moving along some path, we also need to consider that the body is moving forward in time also.
And even if we are not moving, that is, our x, y and z co-ordinates are fixed, yet we move in time as time always moves forward.