because multiples are "many" so you can keep counting on and on and on and you'll keep getting the numbers that add onto
ex: 3, 6, 9, 12, 15, 18, 21, etc. and prime numbers are numbers with only 1 factor ex: 5 is prime because first of all it is an odd number, and the only multiplication equations that go into 5 are: 1 x 5 = 5 nothing else so, it is prime and the factors of 5 are: 1,5 so its prime. (i think i am explaining too much) and also a reminder is: factors= _x_=20 you use this for factors and you add the same number to a number and keep going with multiples.
hope this helps, and sorry if i explained too much.. if i forgot something you need to know, let me know :P