It uses your movements before this to determine whether it feels like your a bot or not
It makes you wait, the biggest issue with bots is they may try to log in say 50 different passwords for example, so if it takes 5 seconds to do each one it makes boting multiple acounts not worth it.
Google uses catchphas with images to choose. They use this to train their own AI or data to sell
Theres a few answrs to this