The mark is used in the measure of weight and as a basis for currency.
Weight[]
The mark is a weight unit equivalent to half a pound. Unfortunately there is no mention of how many ounces make one mark, but likely it is 8.
Currency[]
The mark silver is used in currency unit as well. At least the noble and the floren are based on the mark fine silver. Since a fixed number of these coins (for example, 60 florens) can be minted from a single mark of fine siver, the mark silver is often used for higher amounts.