What does it mean that light "slows down" in glass?

A link to the full video is at the bottom of the screen. Or, for reference: https://youtu.be/KTzGBJPuJwM That video unpacks the mechanism behind how light slows down in passing through a medium, and why the slow-down rate would depend on color. Editing from long-form to short by Dawid Kołodziej