Low-Carb Question

I have been on a low-carb diet for a little while now and have been seeing good results. I have not been doing the ‘carb-up’ days once every two weeks.

Could someone please explain why this carb-up day is important and how I should go about doing it? One of the articles said for 15-20%BF to carb up .75g of ‘clean carbs’. Are clean carbs only vegetables? Thanks!