What is a hot surface igniter?

In a gas oven the igniter does two jobs: it gets hot enough to light the gas, and its current draw is what physically pulls the safety valve open. As it ages its resistance rises and current falls, so it still glows brightly while no longer drawing enough to open the valve — the classic "glows but never lights" fault, and the single most common gas oven repair. Diagnosis needs a clamp meter reading amps, not just a look at the glow.