Videos containing potentially mature content will not be shown to viewers who have Restricted Mode turned on.

• Drugs and alcohol: Talking about drug use or abuse, or drinking alcohol in videos.
• Sexual situations: Overly detailed conversations about or depictions of sex or sexual activity. Some educational, straightforward content about sexual education, affection, or identity may be included in Restricted Mode, as well as kissing or affection that's not overly sexualized or the focal point of the video. 
• Violence: Graphic descriptions of violence, violent acts, natural disasters and tragedies, or even violence in the news.
• Mature subjects: Videos that cover specific details about events related to terrorism, war, crime, and political conflicts that resulted in death or serious injury, even if no graphic imagery is shown.
• Profane and mature language: Inappropriate language, including profanity.
• Incendiary and demeaning content: Video content that is gratuitously incendiary, inflammatory, or demeaning towards an individual or group.

Information gleaned from here:  https://support.google.com/youtube/answer/7354993?hl=en