Summarized List: Status Effects
A prone creature is lying on the ground, rather than standing upright.
While prone, your attacks have disadvantage, and melee attacks against you have advantage. You move at half your speed by crawling along the ground.
Many attacks and effects can cause a creature to fall prone, including other status effects. You can also willingly drop prone at any time as a free action, which may be necessary in order to crawl under something or into a small opening.
You can spend one frame to stand up if you're able.
You drop prone.
You crawl along the ground at half your speed, allowing you to squeeze into smaller spaces. You must be prone in order to crawl.
You stand up from prone.