The sound recordist is, in a sense, the most important member of the sound department. He or she is responsible for what might be called "principal recording"; that is, the recording all sound on set during principal photography or pick-ups. The sound recordist also gives the post-production sound crew a "road map" of sorts for understanding how the sound recordings relate to the visual capture.
A good way of thinking of the sound recordist is to compare him or her to an equivalent in the camera department. He or she is to the sound track what the director of photography is to the visual track. So the sound recordist has the technical task of deploying microphones and other recording equipment, but also is a creative force, in that he or she has to come up with an "aural design" for the episode.