The sum of numbers is when you add them together. It's like when you have some candies and you want to find out how many you have in total. You take each candy, one by one, and put them in a pile. When you're finished, you can count how many candies there are in the pile and that's the sum of all the candies you had.
You can do the same thing with numbers. You just write them down one after the other and then add them up. For example, if you have the numbers 2 and 3, you can write them like this: 2 + 3. When you add them up, you get the sum of 5.