Composition of a signal
I believe that is a simple signal addition:
You have signal 1 on the left with amplitude 1 between time 0 and 1, and amplitude 0 afterward. And then you have signal 2 in the middle with an amplitude 0 before time 1, amplitude 2 between time 1 and 2, and amplitude 0 afterward.

By adding both signals 1 and 2, you get the result signal on the right:
Between time=0 and time=1, the amplitude is the sum of both original signals, Amplitude=1+0=1, between time=1 and time=2 the amplitude is the sum 0+2=2. After time=2 you have amplitude 0+0=0.

At the instant time=1, the resulting signal goes from amplitude=1 to amplitude=2, it doesn't go to zero, apparently.