Pilot Light Problems
Pilot lights produce small flames that are in turn used to light the primary flames of your burners. If the thermocouple malfunctions, however, it will not be able to sense that the pilot light is working. This means that the pilot light might work, but the primary burners will not operate. It could also mean that the pilot cannot function itself, since the thermocouple switches it off automatically without being able to detect any heat.
Gas flow refers to the natural gas flowing from the main line to the oven. When the thermocouple has malfunctioned, it will shut down this gas flow. This is what protects the main burner when the thermocouple is working correctly, but it is also what prevents either the pilot light or the main burner from functioning when the component fails.
If you can manually turn on the gas pilot light on your oven, you can try to light it yourself: If you can light the pilot light manually even when it shuts down without the manual release, you probably have a thermocouple problem. Otherwise, you may need to call in a professional to test your system, narrowing the problem down to a specific part of the oven.
Igniter Keeps Sparking
If you have an electrical ignition to your gas oven, the thermocouple is a part of this ignition system and sends signals to the module to shut down the ignition system once the gas is lit. If you have gas that has lit but an igniter that keeps sparking anyway, this is a sign that the thermocouple needs to be replaced for that burner.