Printf formatting with variable format - what does this var reference?

I am experimenting with printf formatting in bash and I ran across this piece of code:

#/bin/bash
divider===============================
divider=$divider$divider

header="\n %-15s %8s %10s %11s\n"
format=" %-15s %08d %10s %11.2f\n"
width=55

printf "$header" "ITEM NAME" "ITEM ID" "COLOR" "PRICE"

printf "%$width.${width}s\n" "$divider"

printf "$format" \
Triangle 13  red 20 \
Oval 204449 "dark blue" 65.656 \
Square 3145 orange .7

The line that I am interested in is the line that says:

printf "%$width.${width}s\n" "$divider"

What is that variable reference doing? What does it mean?