The 500-word biographical sketch provides new details about this obscure character. Celestina Warbeck isn't actually in any of the Harry Potter stories, but she is referenced several times as Molly Weasley's favorite singer.
In addition to the sketch, Pottermore also posted an audio track for one of Celestina's songs, You Stole My Cauldron But You Can't Have My Heart. Both the audio track and the new bio can be found in the 'Floo Powder' Moment in Chapter 4 of Harry Potter and the Chamber of Secrets. Users will have to register with Pottermore to access the content.
Cauldron is the first song ever posted on Pottermore, but it will not be the last. Universal is ramping up their Harry Potter theme parks and they are using Pottermore as a subtle tie-in advertising effort.