Harem anime is a hugely popular genre and pairs popularly with the ecchi genre. When the main character is a female and surrounded by men, then the anime is referred to as a reverse harem.
The MC is typically a shy guy who has zero prior experience with women. Each of these women has their personality, adding up to the variety of the characters in the anime. All of them are his potential love interests, and as the story progresses, you find them falling for the MC. The primary feature of this genre is that you have one guy surrounded by two or more women/girls. If you are new to the harem genre, let me describe the outline of this genre.