Title text: 'I don't know, why is your beret staying on your head?' 'Staples.'
Beret Guy is hanging upside down a tree, usually something you might have done in your a childhood. As an adult it is not very dignified. White Hat probably made a comment on this and the fact that Beret Guy has his head down. But then Beret Guy gives him an answer, regarding where White Hat's head once came through, and asks him why he is still acting so dignified.
Here, Beret Guy is referring to the process of childbirth. Put in the way he mentions said process, the fact that White Hat came into this world in such a fashion would be very humiliating.
In the title text White Hat replies that he does not know, but then continues to ask why Beret Guy's beret stays on his head, even when upside down (in which gravity would tend to make a beret fall off). Beret Guy explains that he has stapled the beret onto his head.
Such a process is similar to that of closing a wound after major head surgery, using surgical staples. Normally, the stapling of the head would be conducted with anesthetic and removed after the incision has healed, and only an insane person would do this to himself. Doing this the way Beret Guy did it would be very, very, painful. 
Beret Guy is never seen without his hat (although it has been hidden under a green helmet once in 769: War). And later in 478: The Staple Madness it turns out that he likes to staple anything to everything, so maybe also his hat to his head. The staple madness comic may very well be a sequel to this comic.
In the teaser trailer of Indiana Jones And The Last Crusade, Indy staples a hat to his head (or at least pretends to) to prevent it from blowing off, a similar situation to the one Beret Guy is in here.
- [Beret Guy swinging upside-down from tree branch talking to White Hat walking by.]
- Beret Guy: You were once shoved headfirst through someone's vagina. Why are you acting so dignified?
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