What does associative property mean when you’re talking about adding numbers?

Given a string of numbers added together, you may group the numbers in any order you wish and it will not affect the answer you get. This is the basic premise of the associative property for addition. In other words, no matter what numbers are associated together, you will get the same result in the end.
(1 + 3) + 5 = 1 + (3 + 5)
4 + 5 = 1 + 8

The associative property also holds true for multiplication, but it fails for both subtraction and division. Here are the official mathematical definitions for its two incarnations:

The associative property for addition

(a + b) + c = a + (b + c)

The associative property for multiplication

(a x b) x c = a x (b x c)