Well, imagine you want to find out whether a number is prime. You might write a boolean method called isPrime() that takes in a number, then goes through lots of factors to see if it's evenly divisible. If at some point it turns out it is, you would return false, but if at the end it wasn't, you return true.
Then let's say you need to know if a whole bunch of numbers are prime. You don't want to write that entire bit of code a billion times in your program, so you can just be like isPrime(someVariable) and isPrime(someOtherVariable). That way you can reuse code without copying and pasting. It's really quite wonderful.
-Getting ready to go to this guy's house so a bunch of people and I can work on making a game.
-Being pretty excited.