Cash is money. If you've got extra cash in your pocket, you can put a dollar in the tip jar when you buy a cup of coffee.
When you use the word cash, you usually mean actual paper and coin currency: dollar bills and quarters, for example. People often contrast cash with credit cards or checks, and your favorite diner may have a sign that says "Cash only!" When you cash a check, a bank gives you the written amount in bills and coins. In the late 16th century, cash meant "money box," from the Latin capsa, "box."