According to Spotify, there is index called "valence" which can vary from 0 to 1 for every track. Tracks with high valence sound more positive, e.g. happy, cheerful, euphoric, while tracks with low valence sound more negative, e.g. sad, depressed, angry. "Sadder" corresponds to valence of 0, "Happier" — valence of 1.
According to Spotify, energy represents a perceptual measure of intensity and activity. Typically, energetic tracks feel fast, loud, and noisy. For example, death metal has high energy, while a Bach prelude scores low on the scale. Perceptual features contributing to this attribute include dynamic range, perceived loudness, timbre, onset rate, and general entropy.
Assumption is that the more shazams a track have, the more frequently the track has been played in public places, radio or other media channels. Such tracks are not hugely popular, othewise they would be recognized by people without using Shazam application. Also it means the track is "catchy" and "likeable".