If weighted sum >=0 then its output 1 otherwise 0
What actual causes S1 to be either 1 or 0 is all defined by a set of weights w (three for the first neurone, S1, three for S2).
Converting the outputs of the two neurones S1 and S2 into actions is shown below.
The functions for forward, right_turn, etc are defined elsewhere.
At the moment the movement is a bit rough and it is a little simpler to build a version that follows the centre of the line; this approach though, works with thinner lines.
To change the function of the system, change the values in w; for example to produce one that follows the centre of the line just change w (I will leave that to someone to work on). The complete code is shown below.