What does 32 mean in QFN 32? Term is related to IC chip specification
Lets break the footprint name to its parts:

QFN - Quad, flat, no lead.
50P - 0.5mm pitch.
500X500X90 - 5mm by 5mm by 0.9mm
33 - 33 pads, one of them is the bottom pad.
N - This usually means nominal footprint.
D - Variant, this is a manufacturer specific version of this package.

This naming convention is specified in an IPC Standard (I don't remember which one).