It's called a Snap Cap. It is a blank round. The spring is to absorb the pressure from the firing pin. They're used for "dry firing" practice. That's a way to practice without harming the firing pin and also enables you to get a feel for the way the trigger breaks.
Here's a link that may help: OfficerStore.com: Triple K Snap Caps, Plastic
By the way, welcome to the FTF community. Please go to the Introduction forum and introduce yourself to the folks when you can.
If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen. ― Samuel Adams