Prone is a Condition. If you are Prone, you are collapsed on the ground, deck or other surface, minimising your body profile or otherwise knocked flat.
A Target under the effects of Prone Condition causes all Attacks made against them within Adjacency to be performed under the effects of the Advantage Property. All Distant Attacks against them are performed under the effects of the Disadvantage Property instead.
In addition while under the effects of Prone moving costs twice as many MP.
The Prone Condition may be removed by using an Action to stand.