Q: Why can’t the bank keep a secret?
A: It has too many tellers!

Explanation: When you go to a bank, the person who helps you deposit (put in) or withdraw (take out) money is called a teller.  When you share a secret, you tell a secret.  The -er ending on some nouns make that noun into the person who does that thing:
work-worker;
play-player;
make- maker;
tell-teller.

So, someone who tells could be called a teller.  This joke plays with the word tell/ teller and that is why it is funny.