Doujinshi literally means "self-published work", which this is. Think of it like the JP equivalent of "indie".
Doujinshi does not mean "derivative work" or "Japanese porn comic" it's just those particular kinds of doujinshi are the ones most Westerners are familiar with, so people who don't know spread their misinformed assumptions.
Just to add to this excellent explanation, if people wonder why a "Doujinshi" tag and "Self-Published" tag exists at the same time on MangaDex, this is because the Self-Published tag used to be the "User-Created" tag.
The Self-Published tag is generally for non-Japanese (usually user-created Original English Language) works approved by staff for upload here on MD and do not apply to works with JP raws.
All manga (by implication a comic with JP raws) that are not published by an official publishing company (e.g. Shueisha, Kodansha, Kadokawa etc.) and are instead self-published by the author (through e.g. Comiket, Melonbooks, Twitter, Pixiv) are technically Doujinshi. This is one of those.