A boundary microphone is a small type of condenser microphone, positioned near or flush with a surface or boundary, like a stage or table. Boundary mics have what is called a half cardioid pickup pattern (see image below).
This pickup pattern conbined with the sensitivity of a condenser capsule makes the boundary mic very useful in any application where a dicretion is require i.e. no close mic'ing.
Some examples include micing a piano from underneath, stage sound for dance and theatre or conference mic'ing.