A unidirectional microphone is a microphone designed to pick up sound from a single direction.
The opposite of this would be an omnidirectional microphone which itself is designed to pick up sound from all directions. No microphone is truly omnidirectional and no microphone is truly unidirectional, but some microphones come close to one or the other.
Unidirectional microphones work by making sure that their transducer (the part that picks up sound and converts it into an electric signal) is exposed from only one side, which usually means that it is placed inside the microphone and the microphone is open to sound from one side only.
The benefits and drawbacks of unidirectional microphones are obvious. They can protect the singer or player from ambient noise, but they cannot record more than one player or singer at a time.