B7 is a dominant seventh chord made of 4 notes: B, D♯, F♯, A. It sounds restless; it wants to move on, usually to the chord a fifth below, and you hear it in blues, rock and roll, and the V chord that pulls a song back home. It is the V chord of E major and turns up in 2 other keys.
The library above opens on B7 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.