"....the rosette on Harry’s chest kept squeaking their names: 'Troy - Mullet - Moran!'"
-- Irish team souvenir, 1994 Quidditch World Cup (GF8)
The luminous, squealing rosettes are enchanted colourful badges of ribbon, worn to show support for your Quidditch team. The rosettes shout the names of the players until the charm wears off during the next day or so (GF7, GF8).
Notes and interesting facts
From the Web
Writing by J K Rowling on Imgur (Daily Prophet coverage of the 2014 Quidditch World Cup, originally from Pottermore):
MuggleNet: The Best Moments of the Quidditch World Cup
Harry Potter Wiki: Quidditch World Cup