Prone is a condition.

• You grant combat advantage to enemies making melee attacks against you.
• You get a +2 bonus to all defenses against ranged attacks from nonadjacent enemies.
• You’re lying on the ground. (If you’re flying, you safely descend a distance equal to your fly speed. If you don’t reach the ground, you fall.)
• You take a −2 penalty to attack rolls.

You can drop prone as a minor action.

The stand up action removes the prone condition.