Okay but that's also just a very weird way of phrasing it.
As far as I can parse it, it's saying that there is a german word dasein. This word describes a being that is metaphorically in a world of its' own.
Heidegger instead uses the same word to refer to a being that cares about existing in general.

I don't know if that's what his phenomenology actually is, but that's more or less what that sentence seems to want to communicate. Either that or the first bit about being in a world of your own is also something Heidegger says, which would strike me as weird. I imagine "being in a world of your own" is something akin to daydreaming mixed with a little bit of weed-brain "perception is reality, reality is perception bro" but I dunno.

It needs an editor to cut down on the commas and the usage of the words "rather" and "that is". Reeks of college essay trying to up the word count.

Edit: Dasein is a verb meaning "to be there" but it's probably better translated as "to be present in the moment". Incredibly terrible explanation and trying to google it just gives you a bunch of redditors discussing Heidegger.
I have a feeling he's much less incomprehensible if you speak his language.