a persistent dream

Puck suggests that those who don't like the play should imagine they have been slumbering, and thereby find

(..) this weak and idle theme,
No more yielding than a dream (V i 413-414)

Unyielding as a dream. That could mean that the play we have witnessed yields no fruits. Or that it won't give way, won't go away. Just as dreams sometimes refuse to be exorcised or forgotten, no matter how much our rational day-time minds try to laugh them away — nightmares and fantasies.