The concept of Application Programming Interface (API) defines interactions between multiple applications and data exchange in software technology. REST API, an architectural style for building application program interfaces, uses HTTP requests to access and use data. It focuses on users' resources through URLs and ways to access them rather than the actions. REST APIs are preferred due to their use of less bandwidth, compatibility with programming languages, and ease of integration with other websites and mobile devices. API documentation is a crucial design factor that cuts across all application program interfaces, including REST APIs, providing information about services, endpoints, operations, signatures, and responses. It helps reveal an API code's meaning and how developers can use it to achieve a task, giving users a glimpse into what they can expect within the interface before diving in. The documentation also includes interactive tools for live calls, case study guides, guides and tutorials for getting started and navigating the API, and other helpful resources. REST API documentation is important for quick learning, reduced onboarding time, less support calls, increased user experience, and better user satisfaction.