If you want to tell a friend about a fish you caught or a tree you cut down, you're going to tell him WHERE you were ... its position in space, 3 numbers, 'x', 'y', and 'z' ... and also WHEN you were ... its position in time, one more number.
Dimensions are numbers used to describe the location of a point, and the difference in location between two points. With four numbers, you can exactly describe the location of anything, and its distance from any other thing, in space and time.
