they allow your variables to have "slots", meaning that you can store multiple values inside them.
e.g.
integer variable:
i = 4
I can only store 1 value in that variable!
integer variable array:
i[0] = 4
i[1] = 5
i[2] = 7
i[3] = 12
I can store multiple values into whatever slots of the array I desire. you can have as many slots as you like, though I think the editor limit might be 1024.