The word "besides" has two usages.
FIRST USAGE -This is a word highly used in spoken English, used when adding a stronger reason in addition to the reason already given:
-I didn't go to see the move yesterday because I was feeling tired, besides I didn't have enough money.
SECOND USAGE -"in addition to someone or something else that you are mentioning":
-Who else was at the party yesterday, besides you and Anne?
-What do you do when you are free, besides playing tennis?
Not a native and not a teacher