Resilience4j is a lightweight, fault tolerance library designed for Java applications. It provides various patterns to enhance system resilience, such as Circuit Breaker, Rate Limiter, and Retry. These patterns help manage failures and ensure that applications remain responsive under stress. The library is modular, allowing developers to use only the components they need. It integrates seamlessly with popular frameworks like Spring Boot, making it easy to implement resilience strategies in microservices. By using Resilience4j, developers can improve the stability and reliability of their applications, ultimately leading to a better user experience.