You complete a simple task — solve a CAPTCHA, leave a like, or do something basic — and in return you get a tiny drop of crypto.
They still exist today, although now they usually pay just a couple of satoshis.
Do you know when the very first BTC faucet appeared? Back in 2010. It was launched by Gavin Andresen, and the reward was 5 BTC just for solving a CAPTCHA.
Solve a CAPTCHA — get 5 bitcoins. Solve another CAPTCHA — get 5 more. Over and over again.
Those were the days.