An Outpost is a small structure that the Surveyor, Emissary and Explorer vehicles deploy into. It contains command and control systems as well as basic scanning equipment and is used to expand bases by providing ground control.
It takes several seconds for the expansion vehicle to deploy into an Outpost, during which it is very vulnerable. Once deployed, it cannot be packed up again.
Outposts have no defence or armour on their own, and as such must be defended. The later patches remove the build radius around several buildings, including Barracks and Power Plants. This makes rebuilding said buildings difficult without an outpost nearby.