DID is dissociative identity disorder. There's probably a kernel of truth in the condition but has been blown up into a massive online community of attention seeking weirdos - the basic premise is that due to appalling abuse while your identity is forming as a child you learn to dissociate as a coping mechanism (something that definitely happens) but because your identity isn't fully formed you have a persona you 'become' if triggered and this new identity only holds memories of when it's active and the main persona doesn't remember anything about what happens.
This was co opted by the um, tumblr otherkin therin fantasists ? for lack of a better term, and became rife online with people claiming they were systems of complete personalities who took turns piloting the body and all talking to each other in some inner headspace. Because the abuse that's required to not form any real identity is very severe (like life threatening on a regular basis severe) for allllll these people to be suffering from DID has to be wide scale. Which is why the satanic child abuse people have latched on, I assume.
Personally if someone is talking about DID without a ton of caveats I tend to dismiss them