What makes neon signs glow?

Continuous formation of ionised gas atoms due to the application of a high voltage difference across the two ends of the glass tubing. The high voltage briefly pulls out an electron from its bond to a nucleus, but the ion becomes a neutral atom again by capturing an electron which has become free and radiating the energy of ionisation as visible light.

For this process to go on smoothly, the gas in the tube is at much smaller pressure as compared to atmospheric pressure, so that the voltage required to make the gas inside glow is not too high. A lightning flash , which occurs at normal atmospheric pressure requires a voltage difference of 100 keV or more.

The nature of gas inside controls the colour in which the sign glows. Neon gives out a reddish or pinkish flow.