Your legion is your group of friends and allies who fight together in the galaxy. Your legion can trade together, defend each other's planets, and coordinate attacks on targeted planets and enemies.
A legion starts with room for 50 players. Certain legion base modules raise that limit.
A legion base is a central command center for the legion that is similar to a planet. A base can harvest resources and support special modules to benefit the legion. It can be upgraded with energy and other materials. More about bases on the wiki.
New commanders are guided into a legion during the opening tutorial. You can also browse and request to join legions from the Legion tab in the game, or accept an invitation from a legion leader or officer.