"Like Terms" means that you can add or subtract two terms. For instance, you know that you can add and get 5. You were able to add these two 'terms' ( the '2' and the '3') because they are both numbers! However, you might also know that you cannot 'combine' 2 and x. Since 2 is a number and 'x' is not, they are not like terms.
Basically if its two numbers that are the same, For example (-2x and -4x) and have with the same variables you would add them. If they are like this (5x and -2x) you would subtract them because its a negative and positive number and get ( 3x).
