Post Redirect Get (PRG) is one of the design patterns used in web applications to prevent double post or duplicate form submissions which often happens with a page refresh or reload.
When a form is submitted to the server with information, the server responds back with HTML content. When this content is directly displayed on the browser and when the user refreshes the page, the form gets submitted twice. As the content is a response to POST it cannot be bookmarked as well.
To avoid these problems, applications use PRG design pattern which will redirect users to a page instead of displaying the POST response.
More information on the PRG design pattern is available on Wikipedia