There are three spacial dimensions that we perceive daily: the "left-right dimension," the "backward-forward dimension," and the "up-down dimension." Einstein also thought of time as a dimension - the "future-past" dimension. This makes sense when you think about it. When you plan to meet someone somewhere, you tell them the address of the building and what floor you want to meet them. This lets them know where in space the meeting will take place, but if you don't tell them when to meet, they don't have enough information to do them any good. The three space dimensions and one time dimension are collectively referred to as space-time.

First, let's consider only two dimensions of space. In the diagram, you can see that the distance between x and y is shorter than the distance between x and z. If you traveled at a constant speed, it would take longer to go from x to z because you would be moving south as well as east. In effect, you would be splitting up your movement between two dimensions instead of devoting it all to one. Using this argument, Einstein came to a conclusion that everything is constantly moving at the speed of light through space-time. You split up your movement between time and each of the three spacial dimensions. If you are stationary, then you move through time at the speed of light. If you could move at the speed of light through space, you would be stationary in time. This is another reason why it is impossible to move faster than light. Since light moves at the speed of light, it is stationary in time and has no age.