Rounds are a core mechanic in this game as most games you played. They simply represent a single "match". Matches will vary from individual to individual.
First a round needs to know what gamemode it will run. From here the round pulls some RNG parameter to run from the gamemode settings. This decides the RNG preset for all sorts of round attributes.
These attributes include things such as; map, player cap, role availability/rates, objectives, item spawn rates, etc. An admin can override the default settings for each of these for an given round prior to start.
Players should be able ready up for an unstarted game, so they will automatically join when the game starts and will go through the RNG process for their roles, or join a started game mid-round.