As for the signal, imagine a clock face with a hand turning very fast, drawing a circle. Divide the clock into 4 sections (like a graph). I'm going to draw a circle around the very center(origin) of the clock. If I want to send a 1, I shall make the signal stronger, and the circle will get bigger. If I to send a 0, my signal gets quieter and lets the circle shrink.

Back to the hand. I have from the time the hand passes 12 to 3 to send a 1 or 0. I send a 1, so my circle in this section is big as the hand passes. My next bit is a 0, so for 3 to 6, my circle curves toward the center. 6 to 9, another 0, so my circle stays in the center. 9 to 12, I want to send a 1, so it curves out. Each time the hand goes around, I send you 4 bits of info, by changing signal strength. (Amplitude modulation... AM)

What if we want to talk faster? Well, I could speed up the hand(clock speed), I could divide the clock into more sections so we get more bits per revolution of the hand, or many other combinations...

But the principal is the same, we agree that some change or other property of the signal means something. We could use a light bulb, a buzzer, flags... So long as we agree on the system. If we use a wire, it is an electric pulse. If we are on wifi, that pulse is from a small radio transmitter. Fiber optic uses light travelling through special cables.