It looks like you enabled a complimentary session of Pandora Premium, which allows you to interact with an ad in order to listen to content on-demand such as a playlists, albums or songs.
Because these sessions are funded by advertisements, your account will return to your regular service (stations only) automatically at the end of the session.
Pandora offers three ways to listen: free ad-supported Pandora (Stations with ads), Pandora Plus (Stations without ads), and Pandora Premium (Stations + on-demand content). Each option offers access to our industry-leading radio and podcast service.
As an ad-supported or Pandora Plus listener, you can access stations designed to introduce you to new music through the artists and songs that you know and love. You can type in your favorite artist or song and Pandora will do the rest. Music that is similar to your musical tastes will play in your station.