BFF is the acronym for Best friend forever, which means "best friends forever".
The term comes from English, and was popularized worldwide through the social networks, being widely used in Brazil.
Before the BFF, Brazilian girls, usually younger, called their friends "best friend". However, BFF is the evolution of this as it is a "best friend", hypothetically forever.
It is practically mandatory for girls to have a BFF, so as not to feel socially excluded, for example.
Usually, the media tends to talk about BFF ironically, usually when famous people, until then mere strangers, become best friends overnight.
Many critics address the trivialization of friendship or the term "best friend", as fraternal relationships currently appear to be much more superficial and unstable.
The acronym BFF is also present in the music world. The music true friend ("True Friend" in Portuguese), by Miley Cyrus contains the phrase: We sign our cards and letters BFF - "We sign our cards and cards with BFF".
See also about the meaning of best friend.