Because the lines referred to in Act II, Scene 2, are said as an aside, they are private revelations of the thoughts of Polonius. Here in Shakespeare's "Hamlet," Polonius, the corrupt sycophant of the Danish court, attempts to convince Claudius and Gertrude that Hamlet is mad. When he talks with Hamlet their conversation has many non-sequiturs in it, such as the lines in which Polonius asks Hamlet what he reads and Hamlet replies, "Words, words, words" (II,ii,190) and punned insults in which Hamlet says that Polonius might grow old as Hamlet is if "like a crab, you could go backward." So, when the conversation continues between Hamlet and Polonius and the courtier asks, "Will you walk out of the air, my lord?"(II,ii,202), and Hamlet queries, "Into my grave?" (II,ii,203), then Polonius says in an aside, "How pregnant...."
Polonius remarks are much like those of Emily Dickinson's in her poem "Much Madness is divinest Sense." That is, the mind that loses its focus on reality is sometimes better able to discern other matters--perhaps, in much the way intuition works, by a sixth sense. For, Hamlet, perhaps, intuitively senses that Polonius, too, wishes to do Hamlet ill, just as Rosencrantz and Guildenstern do. "How pregnant," how full of meaning, Hamlet's remarks are, like the 'divinest sense" (divine as in to guess) of which Miss Dickinson writes.
I'm not really sure which lines you're talking about because different versions seem to have different line numbers. I am assuming that you are thinking of these lines that Polonius says
How pregnant (220)
sometimes his replies are! a happiness that often madness
hits on, which reason and sanity could not so prosperously
be delivered of.
The basic idea here is that Polonius is saying that only a crazy person would be able to speak as well as Hamlet is speaking in this scene -- only a crazy person would be able to use words so well.
I think that what he means here is that sane people are too confined by what people expect and can't come up with really creative ways of expressing themselves the way a crazy person can.