To “fan and winnow” means to separate the wheat from the chaff. As the wheat falls through the air, it is fanned to blow away the lighter chaff from the wheat grains. Hamlet uses this metaphor to say that Osric has picked up enough pretentious expressions and fancy manners to get him through most situations in which someone is trying to judge whether he has real class. But in fact, Osric’s qualities are no more substantial than the foam on a mug of beer (“yeasty collection”)—if you put them to the test by blowing on them, the bubbles will burst.