Anal douches or rectal douches in respect to sex are about cleansing (rinsing or flushing) the anal canal of feces (poop) by using a tool such as shower enemas, enema bulbs, fleet enemas, and enema bags.
It’s used by people who want to have anal sex and worry about feces in the rectal canal. By the way, anal douches are also used medically by those who want to soften or eliminate hardened stools.
Opinions about safety of anal douches vary from harmless (if done with a proper tool and appropriate rinsing solution) to dangerous, because of greater risks of catching HIV or other STIs (Sexually Transmitted Infections) due to damage of the rectal tissue and internal organic balance.
If one suffers from hemorrhoids, it’s not advised to apply anal douches because it could increase the affliction.