4th Dimesion for Dummies

In physics and mathematics, a sequence of n numbers can be understood as a location in an n-dimensional space. When n = 4, the set of all such locations is called 4-dimensional Euclidean space.

Such a space differs from our more familiar three-dimensional space in that it has an additional dimension, indistinguishable from the other three. This fourth spatial dimension is a concept distinct from the time dimension in spacetime, since time is functionally very different from any of the spatial dimensions; formally, spacetime is not an Euclidean space but a Minkowski space.

Carl Sagan explains the concept of the 4th dimension in easy to understand terms.