300 baud is the speed of the modem. Baud rates started out at 110 and over the years progressed up to 56000 baud. Not too many people actually use modems anymore as DSL and cable have taken over and are much faster than the fastest dialup modem.
To put it into perspective, connecting at 300 baud resulted in text filling up your screen at the rate of about 1 line per second. And we are talking plain text here, no graphics whatsoever.
A BBS (bulletin board system) can be considered prehistoric internet. It's how people communicated electronically before the internet became popular. It was mainly an underground culture for geeks, hackers, crackers, phreakers, etc. There was hardly any porn at all, mostly games and game trading.