AIUI Discord abuses the term 'server' for familiarity to gamers (where 'server' used to mean server but effectively means 'game universe' in practical terms). It's not an open platform like IRC where you can run your own server independent of The Man, as you might assume, it's just another social media company where they use 'server' where normal people might say 'group' or something.
From what I've heard, its main features are low-latency for in-game comms, and being Not Facebook. As ever, whether you need to care about it depends on who you want to communicate with.