Halbwissen is a German noun and directly translates as "half-knowledge". It refers to any given information, or an individual's knowledge on a subject, that is considered incomplete, superficial, or even misleading. In the latter case, the term is sometimes prefaced by the adjective "dangerous" ("gefährliches Halbwissen").
A good example of Halbwissen is the popular American misconception regarding John F. Kennedy's famous speech, "Ich bin ein Berliner". According to the myth, the speech's titular phrase was picked up by the German audience with amusement, as it supposedly translates as "I am a jelly doughnut". The "knowledge" bit of the half-knowledge here is that, indeed, a jelly doughnut is referred to as a Berliner in large parts of Germany.
However, the other meaning of Berliner - "person from Berlin" - is even more common, and the context didn't suggest anything else. Contrary to the myth, Kennedy also used correct grammar, the wordage did not imply he was referring to the pastry. In Berlin, where the speech was held, a jelly doughnut is also typically referred to as a Pfannkuchen, not a Berliner.