Prātimokṣa is a special set of rules that are followed by some people who believe in Buddhism. It's like a big list of things you shouldn't do if you want to be a good Buddhist.
The rules in prātimokṣa are divided into different categories like things you shouldn't do if you're a monk or a nun, things you shouldn't do if you're a regular person, and things you shouldn't do if you want to become a monk or a nun.
Some of the things you shouldn't do in prātimokṣa include: don't kill anything (even bugs), don't steal, don't lie, and don't do anything that would harm other people.
By following these rules, people who believe in Buddhism believe they can live a good and peaceful life.