A denier is an object of currency that you may obtain when trading with people from a tribe or village. After obtaining some denier, you can purchase scrolls, tools, blocks or other useful items. As your reputation increases, you can by better items.
There are 3 different classes of Denier:
Denier (D): worth 1 Denier, and bronze in colour
Denier Argent (A): Worth 64 Denier, and silver in colour
Denier Or (O): Worth 64 Denier Argent, and Gold in colour
Deniers can be obtained through Bandit Houses, trading with people, or killing the population of a Village and raiding the chests. Deniers are also the scale upon which your reputation is measured. Due to the early version of the Millenaire mod in the YogBox, the mod simply using the number of deniers you have in territory considered yours or in your inventory. (You claim land by sleeping in it, it will create a 10 x 10 area that villagers don't open chests in, but they may still build in it). Therefore, when you spend denier, and lose them from your possession, you actually lower your reputation with the village.