A URI, or Uniform Resource Indicator, is a unique identifier used by Spotify to reference artists, songs, albums, playlists, users, and even songwriters within its platform.
Apple Music uses a similar system with Apple IDs.
eMastered uses Spotify Artist URIs to ensure releases are correctly linked to an artist's profile.
Outside of eMastered, URIs can also be useful—for example, media outlets and radio stations might request a Spotify track URI in advance of a release so they can easily add the song to a playlist on launch day.