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.