This block should be placed in loop, and it will set the two motors to follow the edge of a line, based on one light sensor value. [Slow Power] is the speed the inside motor will drive, when the robot is turning. [Fast Power] is the speed of the outside motor, when the robot is turning, and the speed of both motors, when the robot is directly on the edge of the line. The [Thresholds] define the light and dark thresholds for the line.