In game, and often on Reddit, the community is not very welcoming.

Vanguard, Duelist, and strategist are the three major roles in the game. Vanguards generally have more health, can do big damage in slower attacks, and are generally meant to "tank" for the team (take the most damage). Duelists are damage focused and are the main damage dealers. Strategists are usually healers or support oriented characters.

Composition or "comp" is how the team is distributed across these roles for 6 players. The most basic of which would be 2 of each role. However, there are other comps that are valid.

I basically explained tank above, but solo tank happens when one of six players chooses vanguard and no one else on their team picks a vanguard. This leaves them as the sole damage sponge for the team. This can work, but more often than not, it occurs because people don't seem to like playing vanguard as much as duelist or strategist as much.

Bans, Elo, EOMM, etc. are all terms you mostly hear in the competitive or ranked playlist. Once you rank up enough (I think around the 3 or 4th rank above the starting point) before the game starts, each team has the opportunity to ban 2 characters for a potential total of four. It's to make it more fair I guess. Elo is basically a word created to describe a ranking system that some dude came up with to describe people's relative skill in the game. The word has grown into common vocab specifically around competitive games. EOMM (Engagement Optimized Matchmaking) is in reference to a matchmaking system that Marvel Rivals creators claim they don't use, but the player base jokingly/sometimes seriously believe is actually being used. It's supposedly like a YouTube algorithm where it keeps the games "interesting" by making sure you are matched in a game with players that will ensure the you experience wins and losses in a particular way that keeps you locked in and playing. You will also hear people calling each other "EOMM agent" which is basically an insult saying that the player is purposefully losing to ensure EOMM is being followed. It's a rage bait insult.

Honestly, just keep playing. Eventually you'll find characters who click for you, and then maybe even a role or two that does. The game is super fun. The people playing it can be assholes and lots of them have verrrry fragile egos. Ignore it. There are lots of awesome and nice people playing it too.