The “harem” genre in the anime is the environment of the male protagonist (most often) with a large number of girls completely different in image and character. The opposite, more rare, option is also possible: in the center of the plot is a girl. Moreover, it is not necessary at all that the main character should choose one of the ladies surrounding him. For example, friendships are possible.
Note the most popular story: beautiful girls fight for the heart of a guy who rushes between them. The number of beautiful ladies is very different – from 3 or more. “Harem” is usually accompanied by other genres – comedy, romance, fantasy, drama, etc.