So let me begin with the notion of a dimension, the dimension is simply how many numbers that you need to specify a location. So if we think about the surface of the Earth, the equator is one dimensional. You need to specify only one number to specify a location on the equator, namely the longitude. The surface of the Earth altogether is of course 2 dimensional. You need to specify 2 numbers, the latitude and longitude, to specify a location on the earth. The space above the Earth is 3 dimensional. If I want to tell you the location of a hang glider, I need to give you the latitude, longitude and altitude. But mathematically there's no reason to stop at 3:00 dimensions. A 4 dimensional space is simply a space in which you need to specify 4 numbers to specify a location in that space. A 5 dimensional space would be one in which you need to specify 5 numbers, etcetera. In fact, I claim you already know an example of a 4 dimensional space. So let's define an event just to be an occurrence that happens in a small region of space at a definite time. In other words, an event is like a snap of your fingers or a clap of your hands. And now the collection of all events. If I want to specify an event, I need to give you 4 numbers. I need to tell you where it happens in space and when it happens in time.