Outposts are crystal-based totems that spawn soldiers for the faction controlling them and a game mechanic from Hyrule Warriors and Hyrule Warriors Legends. Though they serve a similar function to Keeps, their role is minor and capturing them not required to complete a scenario. There is a certain number of Outposts located within each stage.
Outposts are controlled by an Outpost Captain, accompanied by a small band of grunts. Like Keep Boss, defeating the Outpost Captain allows the outpost they control to be captured, and cause the Captain's small group of subordinates to disappear along with their defeated captain.
From a gameplay standpoint, controlling outposts is generally not necessary or as important as controlling Keeps. However strategically, by controlling them, the player can use them to occupy/distract or harass enemy factions as they move about the battlefield. Of course by the same token, any outpost controlled by enemy factions can occupy/distract or harass allied forces in addition to spawning enemy soldiers. Raid Captains (of either faction) in addition to targeting keeps, will also target an opposing faction's Outposts if they come across them (thus the Outposts presence can be used to distract and/or occupy Raid Captains that come across them).