I'll try to explain as best as possible. This pretty much pertains to actors/actresses who seemingly disappeared (https://www.reddit.com/r/blankies/comments/vgr764/who_is_an_actor_who_doesnt_exist/) from the public's consciousness even if they're actually still actively acting. They're basically not talked about anymore regardless of anything else they do. They're actors/actress who are just "there" to put things in another way.

It could be because Hollywood for whatever the reasons, decided to not push them anymore (https://tvtropes.org/pmwiki/pmwiki.php/HollywoodHypeMachine/ExamplesThatDidntHoldUp). Or, they could never land a genuine break out role or a role that could elevate their careers and profile to the next level.