White Circle

The glowing circle is a white magical ring that acts as a portal to the Kingdom of Tir Nan Og, where the fairies of earth are imprisoned. The Winx first found the ring exploring a farm from Roxy's childhood. The ring was so important that the Wizards of the Black Circle aimed to get it first before capturing Roxy. The White Circle is the oppisite of the Black Circle.

When Roxy finally transformed in a fairy, she fought the wizards greatly but it wasn't until she used the White Circle (though she was scared at first) that the Wizards were defeated in that battle. It opened the portal to Tir Nan Og.

Trivia

 * The White Circle is probably a reference to Fairy Rings, which are naturally occuring rings or arcs of mushrooms. They were associated to many legends, and according to English, Scandinavian and Celtic legends they were caused by elves or fairies dancing.