When you enter a look for a Challenge but you haven't maxed out your Unworn Bonus, you will receive a "Boost your unworn bonus!" popup. You can boost your Unworn Bonus to its Max by using the Unworn Bonus from an item in your Closet (without wearing the item in your look)! 

This is great if you have a garment that you don't want to use for a look, and you want to re-use some garments that you used for a previous challenge without resetting the Unworn value of the items to maximize your Unworn Bonus.

Note that you will only receive this message if you have garments that meet the season of the Challenge and the value needed to max the Unworn Bonus. Items used to boost your Unworn Bonus will no longer be considered Unworn in your Closet.