Prone

You are lying on the ground or on your hands and knees.

You gain Advantage on all saving throws against non-Curse, non-Area ranged attacks.

You are Flat-Footed against all melee weapon attacks.

You take a -4 Penalty to all non-Area melee attack rolls you make, and any attacks you make using a Shortbow, Longbow, or Thrown weapon.

You can only move by Crawling, and cannot Step. If you have a Burrow or Fly speed, you can use those instead of Crawling, if the situation allows for it.

The Ground Fighter Skill Feat can ameliorate some of these effects.