It's a cosmetic addition to your profile used to indicate something. These could be achievements, memberships, access, or attendance. Most are just memes though. 

 
https://image.nostr.build/f809f81930e348921de98332a2e25101160a299474e7586b4efcb1dc74a62cae.jpg 

 That's a lot badges, I never seen them load because makes crash Amethyst 🤣 

 you can draw one on your pfp