Bdim is a diminished chord made of 3 notes: B, D, F. It sounds tense and unstable, and you hear it in passing chords between two others, and the vii chord of a major key. It is the vii° chord of C major and turns up in 1 other key.
The library above opens on Bdim with 14 shapes up and down the neck, starting with hand-checked ones. Tap a shape to hear it, or play it one string at a time.