You can have a look at
collective.ploneboard. Both are simple solutions supporting only the basic forum functionalities: creating forums, creating discussions in a forum and participating in those discussions by creating replies to messages.
Products.Ploneboard is based on
Archetypes, the main content framework up to Plone version 4 (from Plone version 5 on, its main content framework is
dexterity). It is likely not yet adapted for Plone 5.
collective.ploneboard is a partial reimplementation of
Products.Ploneboard on top of
dexterity; it may be a better fit for Plone 5.