Vuex is a state management library for Vue.js applications. It serves as a centralized store for all the components in an application, with rules ensuring that the state can only be mutated in a predictable fashion. Vuex allows developers to easily build complex, large-scale applications in an organized way. It also provides tools to help developers debug their applications, including a time-traveling debugger and state snapshot feature. With Vuex, developers can quickly build powerful applications that are easy to maintain and scale.