What is 'Similar movies'?

I just wanted to understand what is the criteria for ‘similar movies’ new option on the movie’s page screen. The similar movies are very broad and do not relate for many titles.

I can’t find the docs right now, but basically this is what other movies TMDB users recommend to watch. So the same as what you see under “Recommended” when viewing a movie (or TV show) on https://www.themoviedb.org/