To perform the streak test, you rub a specimen on a piece of unglazed porcelain tile. Go to your local hardware store, and look at the backs of their floor tiles. You want a tile that is made of porcelain, and the unglazed back should have a white color.
These tiles are very hard (hardness 6.5), and they have a rough surface. As you rub the mineral across that surface, it acts like a file, powdering up some of the mineral. The color of that powdered mineral is its streak color.
At first, the streak test can be puzzling. You would expect that if you rub a yellow mineral on your streak plate, it would leave a yellow streak. After all, you are just powdering up a bit of the specimen. Why would small pieces look any different from large pieces?