Popularity is a metric from 0 to 100 that roughly describes how popular a song currently is, 0 being the least popular, 100 being the most. The Spotify API documentation puts it best: "The popularity is calculated by algorithm and is based, in the most part, on the total number of plays the track has had and how recent those plays are. Generally speaking, songs that are being played a lot now will have a higher popularity than songs that were played a lot in the past."