what does it mean for a molecule to diffuse down a concentration gradient?

To diffuse down a concentration gradient means that the molecule goes from an area of high concentration to an area of low concentration. This is a spontaneous occurrence.

The difference in the concentration of a substance from one location to another is a concentration gradient!

I believe the blank would be filled with "concentration gradient".

All molecules in any solution that allows them to move freely will move from an area of greater concentration to an area of lesser concentration (a concentration gradient). This is called diffusion, and it happens spontaneously (without energy input).

A classic example is if you drop some food coloring in a glass of water and it spreads out without any mixing - that's diffusion.

If it were moving against a concentration gradient (lower concentration higher concentration), that would require energy and is called active transport.