This term refers to the time between transit vehicles traveling in the same direction on a given route, in a fixed route system. In other words it’s the greatest amount of time a rider will have to wait to catch the next, train, bus, trolley, gondola… Headways in the most humane transportation environments are around 3 to 5 minutes. In some US cities they can be an hour, an hour and a half, or even more. They frequently vary depending on the time of day or day of the week in a given location and are definitely keyed to population size and density in a given market.
To retain effective transit function in smaller, more rural locales, using smaller vehicles that run more frequently may be effective or some rural areas have worked with what is called pulse timing (translated from Taktfahrplan in German), trading off frequent departures for efficient transfer options. Those one hour or even longer pulse timed headways may be workable/reasonable in remote areas, but they should be avoided in cities.
Remember, transit is a key part of active transportation! The ideal is to have many options for meeting your transportation needs, rather than being just locked into one (or none). If you have access to motor vehicle travel -- but nothing else -- that is also a non-resilient and limiting situation.