"Sur" has different meanings.
1- "on top of". E.g. Le livre est sur la table. (The book is on the table)
2- "sûr" means "sure". - J'ai perdu mon livre. -Es-tu sûr? (- I have lost my book. -Are you sure?")
3- "sur" means "sour/bitter (taste). Ces bonbons sont surs (These candies are bitter).
The first two meanings are the most widespread. For English speakers, meaning two and three can be remembered easily if you associate it with "sure" and "sour" (just one letter to change).