Take a look at the top of the page, go to tutorials, find them there.
Heres one:
http://www.hiveworkshop.com/forums/f279/variables-5896/
If you want a summary, a variable is a thing that can hold information. For example, you could make a variable to count how many times a unit has died. Every time the unit dies, you could set that variable to go up by one.
An array is a variable with multiple values. You can have Variable[0], Varialbe[1]... instead of making a bunch of single variables, you can use one, and inside of a loop, you can have the variable with the number of times th eloop has run.. you can keep track of alot of things that way.
Take a look, if your planning on doing any kind of custom map, you should use them.