Bank tellers are people who work in a bank directly with customers. You queue up to talk to them, and they can do simple banking transactions for you, like deposit or withdraw money. Modern banks prefer you use an automatic teller machine or the internet instead of coming into the bank.
Treasure is something very valuable that people keep stored away. Treasure might be a collection of gold coins, artwork, precious jewels or just money.
Something is valuable if it is worth a lot of money. Some things are valuable for other reasons, for example they have sentimental value because they remind you of someone.