A modifier is exactly what it's called, it modifies a code when it's appended. Modifiers give more information reguarding a service that was provided.
I would suggest that you study your modifiers and learn them like the back of your hand. When reading reports there will come times more often than not that you will need to append a modifier to a code or codes to "complete the picture."
When you learn your modifiers you will know when to use them. For instance, a bilateral procedure is done, but the correct code does not specify "bilateral." You would append modifier -50 (bilateral procedure) to that procedure code to indicate that the procedure was done on BOTH sides of the body, rather than unilateral or ONE side. (However, some code descriptions specify a bilateral procedure; in these instances you would not append modifier -50 because it was specifed in the code already.)
-LT, a procedure that is done on the left side of the body, -RT, a procedure that is done on the right side of the body. ...and so forth.