The streak of a mineral is the color of the powder left on a streak plate (piece of unglazed porcelain) when the mineral is scraped across it. The streak plate has a hardness of glass, so minerals with a Mohs Hardness >7 will scratch the streak plate and won't powder the mineral.
Streak can be useful for identifying metallic and earthy minerals. Nonmetallic minerals usually give a white streak because they are very light-colored. Other minerals may have very distinctive streaks. Hematite, for example, always gives a reddish-brown streak no matter what type of luster it displays.