It's a project that's designed to discourage AI scraping. The idea is that every time you access a website, your computer must solve a math problem before it can access the website. For a user, this is trivially easy and wouldn't affect your browsing experience much at all. For a scraper, though, it accesses so many websites so quickly that the time spent doing the math problems starts to add up, and the scraper ends up spending most of the time just doing useless math problems.

As a side note, a similar protocol was apparently proposed in the past for emails to discourage spam emails. Clearly, it was not adopted