"Fring's back" as in Gus Fring. Meaning Gus is back in the show; he was probably the one who left Mike the note that said "don't." It means his operation has been building and he's going to be a major player in season 3.
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Probably wasnt Fring, was probably Fring's bodyguard, the same dude who called Walt when he went to Fring's house to kill him.