I can't seem to find an actual reliable source of what M rating in video game means. Though I did find an article and it says that M Rating is for people older than 17 years old.
That M Rating contains
Even greater amounts of violence of blood and gore and sexual content like sex and nudity
Also OP if you bought this from steam you will get a popup/notification that says this before you can even go to the actual page.
(This Game May Contain Content Not Appropriate For All Ages, Or May Not Be Appropriate For Viewing At Work.)
The developers describe the content like this:
"This Game may contain content not appropriate for all ages. It contains General Mature Content, and may include Nudity or Sexual Content."
then it asks for you birthday just to continue to enter into the actual page on steam.
