Tbh not so many people in the community even know about those. Basically those are Spotify playlists created by riot. They have one for every character except for Kay/O yet. My favourite thing to do so far has been putting on the playlist in the background, depending on which character i pick. Games just feel and hit different, 10/10, i recommend trying it out. If you don't have Spotify, just search PLAY:<agent's name> on YouTube, and it'll give you the whole playlist in one video. My favourite ones so far have been Phoenix and Viper playlists.

Edit: here's the complete list of playlists on YouTube: https://youtube.com/playlist?list=PLP6znfi8EXzk5CM6QwEG1GWajq0u3mEJk