Hi!
We got a little bit about it here in this section
http://www.mathplanet.com/education/algebra-1/factoring-and-polynomials/monomials-and-polynomials
The thing to remember is that like terms are terms with the same variable (or exponential variable). If you have a longer expression that you are about to simplify it can be of help to remember that the sign before each number "belongs" to the number i.e. if we were to add the like terms in the following expression
we can begin by grouping all like terms
and then we can just add all like terms and will then get
if it's hard using the variables think of them as a thing e.g. an apple.
If we've got 2 apples and then buy one more we will have 3 apples. If we then were to give an apple each to 4 people we would realize that we're missing one apple i.e. -x
Hope this was helpful eitherwise just tell me!