The Hebrew word mekes means a tax, tribute, or levy — specifically a proportional assessment taken from spoils of war or income. It appears in Numbers 31:28, 37-41 in the context of the tax levied from the Midianite war spoils for the LORD's portion, given to the priests. It derives from a root meaning to count or number.
The mekes principle established in Numbers 31 teaches that even the gains from battle belong first to God. Before soldiers or the congregation divided their spoils, a mekes — a tribute — was set apart for the LORD and given to the Levites. This principle of giving God the first portion of every increase runs throughout Scripture, from tithes and firstfruits to Jesus' teaching about rendering unto God what is God's. Everything we have is ultimately His; the mekes is simply our acknowledgment of that truth.