If something or someone is petty it means they are small or they think in a small way.

Chris is so petty. He worries about things that aren’t really important.

Petty cash means a small amount of money usually kept in a box in an office. It used for making small cash payments, e.g. stationery, office supplies, tea and coffee, etc.

Mike took ten  pounds of petty cash to pay the pizza delivery guy. Everyone was working late and there was no time to go out for dinner.