ReactJS is a JavaScript library designed to enhance the creation of user interfaces by addressing the challenge of efficiently managing dynamic data in large applications. It introduces an "immediate mode" paradigm, contrasting with the traditional "retained mode" of DOM manipulation, by focusing on rendering states rather than transitions between them. By re-rendering to a virtual DOM tree and performing batched DOM manipulations, ReactJS optimizes performance and reduces the cognitive load on developers, making applications more predictable and manageable. This approach allows developers to treat their code as a black box, simplifying the process of handling changes in data over time. The library was discussed in depth by Pete Hunt, formerly of Facebook and Instagram, during a meetup where he highlighted its benefits and demonstrated its functionality through engaging examples.