It's important to say that a conjecture, in mathematics, it's a conclusion based on certain evidence and it's supposed to be true. In this case, we have the conjecture: The difference between any two whole numbers is a whole number.
When you have a conjecture, the first thing you have to do is to make sure you know the meaning of each part. For example, you have to know what a whole number is. Basically, whole numbers are all the number from 0 to infinite (not decimal, not fractions), it doesn't include negative numbers.
Once you know what a whole is, then you can analyze the conjecture. Notice that the conjecture states that the difference of two whole numbers is a whole number, but if you subtract 2 - 5 you would get -3 which is an integer but not a whole number.
Therefore, the difference 2 - 5 can be a counterexample of the conjecture, proving that it's not true for all cases possible.