Pig-in-a-poke is an idiom that refers to a confidence trick originating in the Late Middle Ages, when meat was scarce but cats were not.
The scheme entailed the sale of a suckling pig in a poke (bag). The wriggling bag would actually contain a cat (not particularly prized as a source of meat) that was sold to the victim in an unopened bag.
A common colloquial expression in the English language, to buy a pig in a poke is to make a risky purchase without inspecting the item beforehand. The phrase can also be applied to accepting an idea or plan without a full understanding of its basis.
If you’re not seeing the video here, you can go to YouTube and hear Nancy Pelosi say, “But we have to pass the [health care] bill so that you can find out what is in it.”