As the name implies, PPC paid ads work on a pay-per-click model. So you're basically paying Google (or another ad network) each time someone clicks on your ad.
The amount you're charged per click on your ad is known as your cost-per-click (CPC).
For example, if you're running a Google Ads campaign with a $1 CPC and you get 100 clicks, your total ad spend will be $100.