Guardian Fairies are the Fairies who are guardians of the magical universe. Once Fairies reach their Enchantix level and graduate from Alfea, they also get the title of Guardian Fairies of the Magic Dimension. After this, they are bound to protect the Magic Dimension and fight its enemies and the forces of evil.
It is possible that once Witches graduate from Cloud Tower they also become Guardian Witches since, according to Griffin, the witches also fight evil, except that they use dark magic while fairies use light magic.
Know Guardian Fairies
The Winx graduate (except Bloom) and become full-fledged fairies and by becoming guardians of their home worlds they are more connected to their magic and it becomes powerful just like them. At the end, Bloom fully complete her Enchantix and becomes a Guardian Fairies.
This season focused on the Wizards of the Black Circle, and the Winx were invited back to Alfea to teach as they were full-fledged fairies. There were more Guardian Fairies like Diana, Aurora, Sibylla and Nebula.