This image here is of a black line follower. Currently both the sensors are sensing white colour ( A '1'). We can say that M1 and M2 are tied to these sensors and are being driven by them. But when this follower will move a little further, the right sensors will encounter black or a ZERO. Stop the right motor on sensing this, and let the Left motor rotate. This will turn the machine to the left following the line.
There could be more modifications to build a better line follower, which I'll introduce in next post, also I'll introduce how to build the required circuit for it.