It means the firing pin, or striker, is driven forward to fire the chambered round by a spring that is compressed by the firing pin itself.
Imagine a typical bolt-action rifle, with the firing pin running down the middle of the bolt, surrounded by a coil spring. When you rack the bolt, the spring is compressed, and pulling the trigger releases the firing pin.
Contrast that with a gun with an external hammer, in which the loaded spring is acting on the hammer which then strikes a blow to the firing pin, sending it forward to fire the chambered round.