The Artist Plan is for a unique user to monitor one artist profile. It works hand in hand with a system of credits.
You will have to use one credit on an artist to access its artist profile. Artist profiles are dedicated pages to find all the data on a specific artist (Overview, Audience, Playlists, Airplay, Charts, Songs tabs, and Song pages linked to the artist).
This credit lasts 28 days. At the end of that period, you can continue to use your credit on the artist X or change the artist you wish to monitor.
Say you want to monitor an artist Y during those 28 days; you will have to stop monitoring artist X to monitor only artist Y. You will still have to wait until the end of those 28 days to make the change.
You can choose the PRO plan if you prefer unlimited access to all artists.