It says right there mate, "Acousticness" and "Valence".

How those are determined is anybody's guess, but they're normalized to a 0.0-1.0 scale.

Edit: From OP's other reply, they are Spotify's proprietary metrics:

acousticness

A confidence measure from 0.0 to 1.0 of whether the track is acoustic. 1.0 represents high confidence the track is acoustic.

valence

A measure from 0.0 to 1.0 describing the musical positiveness conveyed by a 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).