This is about the comparison operators in C++ and making them easy to implement. There are 6 comparison operators in C++; ==, !=, <, <=, >, and >=. If you want to be able to apply all of these to a class, and you have the right type of order, you only need to implement one function which will determines them all. Incidentally this is called a total order, but I won’t go into what that means here.
The tl;dr of this is look at this repository and use a class from there to easily implement custom comparison operators.
This idea is to map the order onto the real numbers (doubles). So you have a function which takes two instances of a class and returns a double. If this function returns a negative number then the first instance is less than the second, if it returns 0 they are equal and if it returns a positive number then the first instance is greater than the second.
This will be easier to see with an example, so here is a class which is just data storage for an int.