It is an offering or deal that is foolishly accepted without being examined first. 'Don't buy a pig in a poke' might seem odd and archaic language. It's true that the phrase is very old, but actually it can be taken literally and remains good advice.
The advice being given is 'don't buy a pig until you have seen it'. In British commercial law it is 'caveat emptor' - Latin for 'let the buyer beware'. This remains the guiding principle of commerce in many countries and supports the view that if you buy something you take responsibility to ensure it is what you intended to buy.
A poke is a sack or bag. It has a French origin as 'poque' and, like several other French words, its a diminutive is formed by adding 'ette' or 'et' - hence 'pocket' really means 'small bag'.
Poke is still in use in several English-speaking countries, notably Scotland and southeastern USA, and describes a type of bag that would be useful for carrying a piglet to market.
A pig that's in a poke might not be a pig. If a merchant tried to cheat by substituting a lower value animal, the trick could be uncovered by letting the 'cat out of the bag'. Many other European languages have a version of this phrase, with most of them translating into English as a warning not to 'buy a cat in a bag'. The advice has stood the test of time and people have been repeating it for at least five hundred years.