FF on Twitter is the acronym for Follow Friday, which means "Follow Friday". FF on Twitter, which is a microblog, is used as a recommendation, people give tips from others to follow another profile on twitter that is very interesting, and worth it.
Generally, FF on twitter is accompanied by the # symbol (called a hashtag in English and "Jogo da Velha" in Portuguese) and that means it's a topic of conversation, and ends up going to the list of most used words, like #followfriday, #FF and etc.
When users FF on twitter, it's so that their followers and followers' friends know another profile, another person. Usually, those who do FF on twitter are famous people or known on social media, as they understand and know many people who might be interesting to others.
When doing FF on twitter, users put something like: #FF @XX, @YY, always followed by @, which means the person's profile.
know more about Twitter and RT on Twitter.