Amoras wrote:
From the wiki:

The Hive Mind is the gestalt collective consciousness of the entire Tyranid race, a psychic embodiment of the Tyranid instincts and racial imperatives to devour and destroy.

It is not a single mind capable of small, conscious thought the way a Human's mind works, but instead it is a grand agglomeration created from the psychic presence of untold billions of Tyranid creatures. It is a nightmarish and unfathomable intelligence that allows Tyranids to coordinate and adapt as if every Tyranid creature is merely a part of a single massive organism, one which is utterly focused on consuming all other life.