B♭/D means “B♭ with D in the bass”: play a B♭ major chord and make D the lowest note. Because D is already in the chord (its major third), this is the first inversion of B♭.
The notes, low to high, are D, B♭, F. The library above opens on B♭/D with 14 shapes that keep D at the bottom; tap one to hear it.