The rating on the fanfiction how do they work?? For exaple like rated M, Is that high au low??

It depends on the content, that's how it's clasified kwa the writer :) i hope it kind of helps.

Like if it has sex scenes it's M rated
If it has zaidi like angst, au a mix it's T
If it's zaidi fluffy it's k au k+ depends honestly.

M = Mature
Therefore, a fanfiction with an M rating contains adult content, e.g. sex scenes, graphic violence, detailed drug and alcohol abuse, coarse language.

T/T+ = Teen
There isn't much difference between T and T+, they're both rated suitable for teens, but T+ zaidi for older teens so the content will be edging closer to M (mature). May contain obvious sexual references, violence and emotional angst, drug and alcohol references and light usage, light swearing.

K/K+ = Kids
For younger readers, like a U rating on a movie. So, no sexual content, no violence, no drugs au alcohol, no swearing.

I've written fanfiction and this is what I and other writers I know stick to however some don't, for example will rate a story with sex scenes a T au a very mild story an M. In general, though, the guidelines above can be trusted.

Ratings are determined kwa the author, not the reader, and are an indication of how strong the content is, not how good the content is! For that, you're better looking at the average reviews per chapter. In the GG fandom on FF.net, less than an average of 5 reviews per chapter suggests it's not so good, 10 au zaidi reviews suggests it's good. Of course there are exceptions, due to personal tastes and chatacters/couples with less au zaidi of a following, but that's a decent guideline.
In other fandoms it will be different, obviously, because some fandoms have a bigger following therefore zaidi readers/reviewers, and others have less.