It means BYOB but fun is substituted for booze. This is done because alcohol often starts the fun. The lone Sober person at a party does not usually have any fun.
Fun is also substituted as a code word so that minors don't have to worry about adults figuring out what is going to go on at a party.
The fact that it sounds nerdy and innocent makes it ironic.
Bring your own fun to the party saturday.
Should have brought some fun.