The following figure shows the concept of a line follower robot:

In the preceding figure, we have two conditions. The first is when the left sensor is over the line and the second is the same for the right sensor. The black line is the track that the robot should follow. There are sensors represented by blocks. Let's consider the left-hand side of the preceding figure.

Initially, both the sensors are over the white surface that is in position 1, as shown in the preceding figure. Next, consider position 2 on the left-hand side of the image. The left sensor comes over the black line and the sensor gets activated. Based on this, we intercept sensor readings and relay control signals to the motor driver. In the preceding case, specific to the left-hand side of the image, we get that the left sensor detected the line and thus the robot should not go forward, instead it should take a slight left turn until and unless both the sensors return the same value.

The same is the case for the right-hand side of the figure where the right sensor detects the black line and the motion execution command is triggered. This is the case of a simple line follower robot where a single colored line needs to be followed. Things get a bit different with multiple colored lines that need to be followed.

Now that we know the exact process of following a line, we can now focus more on how the robot executes turns and the robot structure.