It is sort of visual but it's not really to do with general appearance. Streak is the color of the mineral when it is powdered down or rubbed against a hard surface. It is different from the color of the mineral in its solid form, which can be affected by impurities, coatings, or weathering. It's one of the more reliable ways to identify minerals, especially for minerals with metallic luster.
You need a streak plate to test a minerals streak colour, it's basically a piece of unglazed porcelain with a rough surface. You can also use the back of a ceramic kitchen tile. You simply drag the mineral across the streak plate and observe the color of the mark it leaves. Some minerals have a white or colorless streak, while others have a distinctive color. For example, hematite has a red-brown streak, pyrite has a black streak, and gold has a yellow streak. You can use it to help filter down possible candidates for id purposes (it's a combinable choice in crystal properties in the crystal guide pocket edition app).
