Once upon a time, when people discussed gas furnaces, they often talked about the pilot light. This flame burned through the winter in the furnace, ready to ignite the burners as soon as gas started to flow to them. Sometimes the pilot light would go out and need relighting—and this was often Troubleshooting Step #1 on lists of what to do if a gas furnace stopped working.
But now the question people are asking isn’t “how to light a gas furnace in Fresno, CA” so much as wondering what happened to the pilot light. Some models may still use them, but not many. The pilot light has gone out of fashion, and for some good reasons.