Like a pat on the back or a round of applause, Bravos are a way to cheer on your fellow players. Every time you attend an event, you receive three Bravo tags with your character sheet. These are useless to you, but can be given to other players who do something to make the game more fun.
For instance, you might give a Bravo to a player for a great costume, immersive roleplaying, exceptional storytelling, being helpful, or even cracking a hilarious joke. You can't claim your own Bravos, more than one Bravo from the same player in a single event, or Bravos from an event you did not attend or write content for, but otherwise there's no restriction on who you give them to or why. They are a tool to incentivize the behaviors you believe make the game more enjoyable.
Each Bravo you claim increases your lifetime Bravo score. So long as you claim them within two weeks of the event, Bravos also work much like raffle tickets, giving you one chance to win a rare item generated for that event. Likewise, each Bravo you claim in a year is one chance to win a grand prize generated at the end of the year.