They are videos of manga chapters (some full, some partial) with voice actors reading out the dialogues -- IT'S FUN! It's not like I'm watching a bunch (I'm really just going with the YT recommendations here) but recently I'm kind of in love with Kimura Ryohei-san's voice in this one XD... yeah, you probably guessed it right, the recommendations are of the isekai-themed kind of stories, and they seem to keep on coming after I watch one and more (good job, YT!).

Besides getting to see/hear to some good acting performance, it doubles (triples?) as Japanese reading and listening practice, which is why I say it's fun~ 

Usually this type of videos are region-locked so I'm going to watch as much as I can before they realise they need to lock them!