The voltage propagates as a wave along the wire. The wire (or, transmission line) is modeled as a distributed inductance and capacitance.

With an unterminated transmission line, the wave bounces back toward the source with opposite polarity. If it's terminated at the far end, the wave is absorbed at the end.

Here's a video with a visualization of wave propagation on a wire, unterminated and terminated: https://www.youtube.com/watch?v=ozeYaikI11g

The speed of wave propagation is some fraction of the speed of light, *C*. For a low-loss coax cable, the speed will be about 0.8 *C*. For a PCB trace it's more like 0.6 *C*.