It is a common fact that you can describe the position of a point in space by three numbers, called coordinates. As an example, you could say that a point in a room is 9 feet from one wall, 10 feet from a second wall, and 5 feet up from the floor. You can use any three coordinates you want, but it is best to use ones that make sense.
You wouldn't describe where the moon was by telling the number of miles north and west of the Empire State Building and feet above sea level, although that would be a good way of describing the position of a helicopter flying over New York City.
An event is something that happens at a particular point in space and at a certain time. Because of this, you can describe an event by giving four coordinates. As I said before, you can use any three numbers you want to tell where it happened and a fourth number to tell at what time it happened.
These four coordinates of an event describe the event's position in what is called "space-time," also known as the space-time continuum.