Some expressions only contain one operation and are pretty easy to simplify (operations are just things like addition, subtraction, multiplication, etc.). For example, to simplify the expression 2 + 3, you just add 2 and 3 together and get 5 as the answer.
What happens when an expression contains more than one operation? Things get just a little more complicated. Does it make a difference which operation you do first? Let's look at an example with two operations: addition and multiplication.
Will it make a difference if you add first or multiply first? Let's see.
If you add first, you end up with 25 as the answer. If you multiply first instead, you get 17 for the answer. Which one is correct?
Mathematicians discovered this sort of problem hundreds of years ago. To avoid confusion and multiple answers, they developed what's called the "order of operations." Basically, everyone got together and they all agreed to simplify expressions in the same order. This way, expressions will only have one answer and there won't be any confusion about which one to do first.