The document discusses RxSwift, which is a library for reactive programming with Swift. It combines ReactiveX with Swift by providing Observables and Observers. Observables allow data streams to be observed and manipulated through operators like map, filter, etc. The document provides examples of using RxSwift to validate a password field by observing text changes and mapping valid/invalid states to display feedback. It also shows an example of observing a nickname field to call an API on valid input. Overall, the document introduces the key concepts of RxSwift like Observables, Observers, operators, and provides examples of validating user input fields reactively.