Introduced in Generation 3, ribbons are flavor items rewarded to one's pokemon for completing various milestones. They represent a multitude of things such as contest wins, entering the hall of fame, or having max friendship, and many other feats. There are 115 ribbons that exist throughout all of the games from Generation 3 onwards as of Generation 9. A ribbon master is a pokemon that has acquired every single ribbon (and mark in Generation 9) available to it throughout each generation. The total amount of ribbons available to each pokemon varies thanks to things like battle clauses or the dex cuts of Generation 8 and onwards, but generally this leads to a pokemon sporting an impressive display of its accomplishments throughout the regions. Ribbon mastering is a great way to give extra love to your favorite pokemon and can be a fun new way to engage with the game on your next replay. A small community has formed around this practice, and they created a bunch of resources one can follow to guide them along. I'll link them somewhere on this site at some point, so be sure to give it a look!