You are lying on the ground.
Effects
- Movement is reduced to crawling at 1/4 speed
- Attackers in melee gain advantage against you
- Ranged attackers at distance have disadvantage against you
Standing Up
Standing from prone:
- Costs your Primary Action
- Provokes a reaction unless you also spend your Move Action, or you have the Acrobatics ability
Tactical Considerations
Being prone provides cover against ranged attacks at distance, but leaves you vulnerable in melee combat.