Hallo and welcome!
Blaze and van Leigh said the useful practical stuff. I just want to add: sims are what we call the training exercises the characters do. So, in-character, the crews go to the equivalent of flight simulators and run exercises, where even if the ships get destroyed there are no dire consequences. Missions are in-character “real”, where the crews are on board ships, with orders, and where what happens has repercussions in the game: if a ship gets too badly damaged, it will need to go to dry-dock to be repaired; if a ship is destroyed, it’s gone; and the crew will have to eject to life pods and hope to be picked up by another ship.
So, sims tend to be a good opportunity to switch around to stations people are less familiar with and want to train in, because any mistakes won’t do any harm; while on missions, ideally you want people to run the stations they’re most familiar with, so nobody dies…
Each shift we usually start off with a sim or two, and then we’re briefed on a mission. If we complete the mission with time to spare, we might do another sim before concluding the session.
But the best way to understand it is to do it, so do join us!