If you’re not of a certain age, you probably have never actually used a BBS; they still exist and you can still visit them but it’s hard to cover how big of a deal and how pervasive these systems were in the 80s and 90s. In lots of ways, they’re the spiritual predecessors of modern federated services: individual instances run by a single or small team of admins that had their own culture, appearance, and rules which were (often) connected by discussion forums, usually called message bases.

A good place to start would be one of the several great video series: Jason Scott of textfiles.com and archive.org released a BBS Documentary that covers the technical, cultural and ongoing impact that the BBS had.

Also, a slightly more recent take is Back to the BBS which covers similar, but different perspectives on BBSes and their lasting impact.