What does "Online Interactions Not Rated By the ESRB" mean?

ESRB ratings only address content created by the publisher, and cannot consider content that is created or introduced by individual players when playing the game online. ESRB-rated games that can be played online with other players and which enable exposure to user-generated content are required to display an Online Rating Notice on the game's packaging (if it exists), as well as in the opening screen, which states: "Online Interactions Not Rated By The ESRB." This notice is intended to warn about possible exposure to chat (text, audio, video) or other types of user-generated content (e.g., maps, skins) that have not been considered in the ESRB rating assignment.