A wavefront is the crest of the wave. When you go down to the beach, and see those things called waves, the front is the whole line that is at the same height.
In electromagnetics, it's the same thing. It's the points that are at the same height. In your diagram, the black curves represent a unit of cycle, and the two waves through b and c can either add to each other (to get double height), or can be out of phase (so b=-c and it's nothing).
Basically, S1 produces a single source, and there is only one set of ups and downs. S2 uses two points of S1's radiation to make two points that are using the same cycles. They don't have to be in the same phase, just that their period is the same. So b and c produce waves, and these intersect, and add together based on how they are in phase.
You can see a result by draging a sine wave over another one. The crest of the sine waves are the black arcs, the intensity is found by adding, eg one wave shifted by 0.5 cycle to the other. They cancel out.