Ignitors work by creating a spark which ignites the incoming gas. This spark is often made between two electrodes, although for some systems there is a single ignitor point that is quickly heated up by a burst of electrical current. With each click, the ignitor is producing energy to try and ignite the gas. Some perform this process automatically, while some burners have an ignition setting on dials that you have to hold.
If your ignitors keep on clicking on your gas burner, but your burner still operates correctly, then you don't have anything to worry about. A series of rapid clicks at the beginning of the burner sequence is expected, since the ignitor needs to work for brief period before the gas has built up enough to ignite. Inside the oven compartment, intermittent clicking may occur as the burner fires up, increasing the temperature over several minutes to the proper degrees. However, if the ignitor does not click at all, or if the ignitors click even after the burner is on, this indicates a problem.
If the ignitors do not click at all or only click once before stopping, you probably will not be able to light your burners at all. This is a sign that something is wrong with your ignitor itself. It could have burned out or deposits could have formed on the electrodes to keep them from working. Cleaning and replacing ignitor components should be able to solve this problem.
If your ignitor will not stop clicking, this is another sign that something is wrong with your oven. If the burner will not light, the gas flow is probably stopped or clogged. If the burner lights but the ignitor keeps on clicking, this is a sign that the ignitor is not sensing that the burner is working or that your dial is stuck on the ignition system and cannot move on.