Demerera sugar vs. brown sugar
Demerara sugar is a sugar made from reduced, crystallized cane juice.  That is, it is a form of brown sugar with the molasses left in during the refining process.

Regular brown sugar is actually made from pure white sugar, with molasses added back in for flavor.

They will have similar performance and flavor profiles so you can substitute freely, although demerara sugar may have less of a pronounced molasses flavor.  Your best substitute is light brown sugar.