What does #FF mean on Twitter?

[Jane]

I'm new to it and keep seeing people putting this

[acco macco]

Follow Friday

Usually a tweet will read "Check out @soandso #FF". This means that the tweeter wants you to check out and follow the person mentioned in the tweet.

[john]

ff is the same as #followfriday
Follow Friday is where a Twitter user recommends to their followers whom they should follow.
However, it is often done in reciprocation to a recommendation instead of a recommendation in itself.

More info here http://mashable.com/followfri/

[Insidius]

The # is a way to create a searchable tag and the FF means Follow Friday. So if you want to find people's recommendations of who to follow you would click on the blue hyperlinked #FF in their Tweet and then Twitter would show you a list of tweets that have that hashtag in them.

[Robynn]

Follow Friday . So on Friday you put that and mention a twitter name and thats how other will gain followers
E.G.
Follow the best person on twitter has good tweet @Twittername #FF

Bare in mind this is only of Friday