Unconscious is a mental status effect that can be inflicted on XCOM Soldiers in XCOM 2.
Unconscious units are unable to move and act for the rest of the mission, similar to when a soldier is Bleeding Out. Unconscious units can be carried by other soldiers to reposition or evacuate them.
Unconscious units that are left behind during an evacuation are considered lost, though unconscious units that are not evacuated by the end of a non-evacuation mission will be revived and can be selected again for another mission.
Any attack that can Stun the target (e.g. a Stun Lancer's Stun Baton or a Berserker's Devastating Blow) has a chance to knock the target Unconscious, which increases as the target's health decreases. The Subdue ability, which is used on enemy VIP targets, also knocks the target Unconscious. This is the only time an XCOM Soldier is able to knock an enemy unconscious.
Unlike dead or bleeding out soldiers, Unconscious soldiers still retain their health bar and can be damaged and healed accordingly. While aliens will never target a unit that is unconscious, area of effect attacks will still damage them. For this reason, if you are not able to revive an Unconscious soldier (and do not need to carry them to an evacuation point), the safest thing to do is to leave it behind, so it won't be hurt when your active units are targeted.
A bleeding out soldier that is Stabilized will be rendered Unconscious with 1 health point.