The Joke:
I got fired from my job at the bank today. An old lady came in and asked me to check her balance, so I pushed her over.
This is a classic play on words (a pun) involving the word balance.
The Two Meanings
The Banking Meaning: The amount of money currently in your bank account. The old lady wanted to know how much money she had.
The Physical Meaning: The ability to stay upright and not fall down.
The Twist
When the lady asked the teller to "check her balance," she wanted to see her bank statement. Instead, the teller treated it like a physical test—he pushed her over to "check" if she could stay standing.