It is a property of most materials that they will give up electrons much more easily when hot - meaning temperature. This was common nuisance with tube devices that had to be turned on to warm up for several minutes before they would begin working.
Cold cathodes typically employ a much higher discharge voltage so they can emit adequate electrons at typical room temperatures. Alas, this is much less efficient.
Hot cathodes have a heater (basically a little tungsten filament with a special coating) that emits lots of electrons to travel through the gas in the tube and make it glow.
Hot cathodes are quite a bit more efficient. About the only advantage of a cold cathode is the instant start..
