Some references throughout the series

Most of the time there is no nudity but certain episodes have more sexual subject matter than others. the older episodes have more sexual references.

He evaluates video games which have gore in them (like FNAF)

The game he mainly talks about and got famous for is about a serial killer who killed his own family and ran a pizza restaurant made for murdering children. The entire lore is pretty horrific and some games he talks about are more horrific

Some of his videos have blood in either the thumbnail or the video. (Sometimes both)

Some games he talks about is rated M

Fuck, shit, dick, ass, hell, damn, goddamn, piss, and crap all have been used, albeit extremely infrequently

Infrequent references to Alcoholism and even more infrequent references to both hard and soft drugs.

He theorises horror games.

Many videos are rather disturbing because of the games he reviews and the theories he makes about them. However, not all the videos are like this and they usually take a rather playful approach to explaining the theories/evidence, and the thumbnails are usually a good heads up to how disturbing the video will be.

though most episodes are fine. some of the horror episodes (particularly FNAF) there's a lot of talk about hopelessness, child murderers, and some episodes explore some of the more dark limits of the human body (IE how much force it takes to shatter a skull, or rip off an arm.) but overall its not too bad if you stay away from the FNAF episodes

some of the horror episodes do have content which may be disturbing to younger children such as serial killers, ghosts, and people dying in painful ways but even when these kinds of episodes do come up MatPat often addresses them with an overall light tone and often makes jokes

In some the earlier FNAF episodes, Matpat includes some of the jump-scares from the games.