It denotes their community level as seen in this list: Community WIKI: the ultimate community guide! 📖
Specifically, the grad cap is for members.
" - Member
You’ll be promoted to a Member level after you have visited and contributed more to the community.
By reaching a Member level you are granted additional abilities.
For example - you can create group personal messages, start your own personal space, and like up to 300 posts per day.
You will get a bronze badge when you reach a Member level as well as a special graduation hat avatar flair. This will show next to your avatar when you post!"