It’s a Group of players who share similar interests or work on projects together. Joining unions is completely optional, no player is ever forced to join one. If there is any union(s) or player(s) constantly harassing new players to join their union report them to staff as this is not allowed.
It is recommended that brand new players do not join one right away. Wait until after getting or rank (4 days and 9 days of /playtime respectively), get used to the server’s mechanics and the community. It’s far too common for a new player to join a union and leave the server shortly after