Commanders in an army are the ones who give orders to the lower-ranking soldiers. These orders are represented in Star Wars: Legion, by you moving the figures around the board, but also by the Command Cards. They determine who will have priority during a turn, being played at the start of each. There are some that are neutral, and can be used by either army. But some are specific to the Dark or Light sides. You get a look at several of these in this preview.
From the post:
In Star Wars: Legion, each player will have a hand of seven carefully chosen command cards. These command cards must be selected before the game begins, as part of building your army. (We’ll take a closer look at army building in a future preview of Star Wars: Legion!) At the beginning of the round, you’ll secretly choose one of your command cards and reveal it at the same time as your opponent. These command cards determine which player has priority for the round and how many orders you can issue to your units