Grade is a measurement of the metal content of ore.
The grade of precious metal ore is usually measured in troy ounces per tonne or grams per tonne. The grade of ore bearing other metals is usually a percentage (the weight for weight proportion of metal in the ore).
Mill head grade is the grade of mined ore fed into a mill, as opposed to the grade of the ore mined.
The grade of ore from a mine changes over time. A lower grade will (other things being equal) mean a higher cost per unit weight of extracted metal. Costs are also influenced by other factors such as the costs of the actual mining of ore itself.
The most important factor in the profitability of a mine is usually the price of the metal (or other commodity) that it produces.