OK my paper says the numerical expression 5+12 represent the sum of 5 and 12. I don't understand

That literally means the numbers '5' and '12' written like this: 5 + 12

represent the sum of 5 and 12, which is the same as 5 + 12 (because the sum is when all the numbers are added up).

So it's saying 5 + 12 equals to 17.

Do you get it?

The sum means answer of a addition problem and the answer of 5+12 and the sum of 5 and 12 are the same thing