Your pool pump impeller can be thought of similarly to a boat or airplane "propeller". Your impeller is located inside your circulation pump in your pump room. Impellers are the heart of your entire circulation system. Attached to a motor on a shaft, they spin furiously "sucking" (simplifying here) water in from your pool and discharging it (pushing it out) through your filters, heater, UV if applicable, chemical application system, and eventually back to your pool. Pool impellers are typically made of bronze and are sized and trimmed specifically for your exact motor and to achieve your specific flow (GPM) requirements to meet your required turnover rate. Impellers can deteriorate. If your water quality parameters are not consistently maintained in the proper ranges, the water can actually degrade and disintegrate the fins or blades on an impeller. Or, if you have a blockage on either side of the pump for an extended period of time or an oversized pump, the impeller will continue to spin the same trapped water without moving it and cavitation can occur. Cavitation often sounds like rocks rattling around in your pump and occurs when the impeller begins to boil and super-heat-to-steam the water that is trapped in the volute around the impeller. Cavitation weakens and damages the metal in the impeller.
If you ever open up your pump for maintenance and your impeller looks like the picture above, it absolutely must be replaced. There is little chance an impeller in this condition can successfully circulate water to meet your turnover needs.