The property described by that mathematical statement is:
The associativity of addition.
The operations on the left side of the equals sign are done in the order they appear, from left to right.
The operations on the right side are done using the associative property, first doing the operations inside the parentheses, then adding the remaining terms.
And the statement shows that for addition, the order of operations does not matter as long as you associate in the proper way using parentheses.