Most apps need an interface that lets users navigate their app. Navigation can mean many things, such as:
Including a header, sidebar or footer with buttons or links that take you to a different part of the page or to another page
Links in text elements that takes the user to an external page
Buttons that take the user to a new section, such as the next step in a form
Single-page and multi-page navigation
App navigation are split into two types:
Single-page apps are apps that navigate without leaving or reloading the page, by dynamically hide and show containers and elements based on the user's actions.
Multi-page apps are apps that navigate by sending the user to different pages.
What method to use is not necessarily a black-and-white question: most applications use a mix between the two, where some features are placed on a larger single-page navigation page, whereas others are placed in isolation on separate pages.