Reflection occurs in electrical signals. When voltage pulses, or bits, hit a discontinuity some energy can be reflected. If not carefully controlled, this energy can interfere with later bits. Remember, while you are focused on only 1 bit at a time right now, in real networks you will want to send millions and billions of bits every second, thus requiring you to keep track of this reflected pulse energy. Depending on the cabling and connections that the network uses, reflections may or may not be a problem.
Reflection also occurs with optical signals.
Optical signals reflect whenever they hit a discontinuity in the glass fiber, such as when a connector is plugged into a device. You can see this effect at night if you look out a window. You can see your reflection in the window even though the window is not a mirror. Some of the light that is reflected off your body reflects in the window. This also happens with radio waves and microwaves as they encounter different layers in the atmosphere.