Onboarding is the guided process of familiarizing users to your application and making sure they can fulfil their intended use (e.g. signing up, opening an account, operating a service etc.). A good onboarding experience makes sure your potential user never feels lost or confused throughout the process and increases the likelihood of retention. In practical applications onboarding translates to a series of screens and/or interactive elements that navigate the user through a series of steps in order to complete a predefined task.
Onboarding is really important in mobile design for the following reasons. It enables users to create new profiles and register through the app quickly and seamlessly. An app can have many features and functions that are unknown to a first time user. Using onboarding techniques you can educate new users by revealing app functions or guiding them in specific tasks. It is much easier to familiarize with a new interface through a well-crafted onboarding process. Essentially onboarding is like a warm introduction to your mobile app. You want to strike a balance between the level of information and engagement that the user requires in order to offer a great experience.