Efficiency-cores are specialized processing units designed to handle less demanding tasks in a computer system. They consume less power compared to traditional cores, allowing devices to run longer on battery while still performing everyday functions like web browsing and document editing. These cores work alongside performance-cores, which are optimized for more intensive tasks such as gaming or video editing. By distributing workloads between efficiency and performance cores, systems can achieve a balance of power and performance, enhancing overall user experience without sacrificing energy efficiency.