Rocket igniters are used to start the combustion of solid state rockets. They have to work perfectly and in sync with the other igniters in the system if there are multiple engines. These crucial aspects can make or break a launch, which naturally means that the have to be engineered with incredible predictability. High-grade engine igniters have failure rates that are essentially zero.
If you want to learn a little more about how these igniters work and just how they can be so reliable, take a look at the video below along with some incredible slow-motion shots of the entire process.