G/D gives the bass its own instruction: keep the upper chord, but make the lowest note. The slash separates those two jobs.
D is already the upper chord’s fifth, so this is second inversion. Hold the upper chord steady and move only the bass. That low line can connect nearby chords without jumping root to root.
Also written G over D or G with D in the bass.

In which keys. Every note of G/D belongs to C major (the V), D major (the IV), E minor (the ♭III), G major (the I), A minor (the ♭VII) and B minor (the ♭VI).
Read G/D as the triad in second inversion, with its own fifth in the bass.
Contains G/D and adds second and sixth.
Contains G/D and adds second, sharp eleventh, sixth and major seventh.
Adds second, fourth, sixth and flat seventh. The flat seventh pushes the sound toward a dominant chord.
Adds second, sharp eleventh, sixth and flat seventh, for a dominant sharp-eleventh color.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.