When you throw the light switch in your home, current must flow into the cable before that current reaches the lamp. So you have a traveling wave front of voltage and current flowing down the cable and these wave fronts then meet the lamp.
Before they meet the lamp something must have defined the current wave front i.e. some impedance must have been immediately present so that a current could start to flow (after all the current doesn't meet the lamp for a few nanoseconds later).
The thing that defines the initial current is the cable - it has a characteristic impedance and that impedance defines the initial current flow.
So you have voltage and current traveling down this cable. Volts x amps = power and if the power reaching the lamp (or load) isn't impedance compatible with the load then some power is reflected back up the cable.
Of course, within a few more nanoseconds this resolves - the various waves are sent, returned, modified etc. and eventually settle down.
Now, as a thought experiment, imagine your cable was thousands of miles long - say 100,000 miles and, imagine it was lossless. You throw the switch and about one second later you see the lamp glow at about half brightness. One second later there is a reflected wave returned to the switch which causes a bigger current to flow and one second after that the lamp glows a bit more like it should. This continues back and forth until the lamp settles down to its normal constant brightness.
Now imagine you were transmitting high speed data and you didn't terminate the cable properly or you used the wrong cable. Can you imagine what would happen?