Did you possibly mean to write 你们 instead of 你门? I am asking because you mentioned Duolingo.

If you mistyped and it is 你们 that just means "you" but plural. You can think of it like 他们 = they, 我们 = we etc.

But 你是 is more comparable to "am/is/are" of English. This is not the full extent since it is not always used, sometimes replaced with 很 or omitted. Best way to learn about it would be to look up a Youtube video or the Chinese grammar wiki.

I highly recommend getting HSK books and going through them with Lichao Chinese on Youtube or ZerotoHero on Teachable. Former is free, latter is not, both are good content that follow the structure.