Literally: blind – turtle – float – tree
Alternately: An incredibly rare event; something nigh-impossibly difficult to find. By implication, something you really should take advantage of while you have the chance.
Notes: 盲亀 is, as the characters say, a blind (sea) turtle. 浮木 is not just a “floating tree,” but specifically driftwood.
This phrase is derived from a Buddhist allegorical story about a blind sea turtle who only comes to the surface of the ocean once in a hundred years, and upon surfacing happens to stick its head into the only hole in a floating piece of driftwood. It is said that one’s chances, in the endless cycle of Samsara, of being reborn as a human and gaining entry into the Buddha’s teachings is as rare and difficult as it is for that turtle to rise and poke its head into that hole in the driftwood… so now that you’re here and listening, I guess you’d better take advantage of your amazing good fortune and try to free yourself from attachments to the illusory material world!