C++please explain the code, thank you!

Adding BIG Integers In C++ an int is in the range 0 to 65535. But what if we need to add larger integers? Say we want compute the sum 2345566777844567+ 9999988777765768009998. You task in this assignment is to make this happen. Write a function string add(string a, string b) where a and b are strings representing integers and the function add(a,b) returns a string representing their sum. Strings a and b can have a maximum of 100 characters. Write a main program that loops asking the user for the two numbers a and b. The program should terminate if the user enters done what asked for number a