The Spring MVC form tag library is a set of custom tags that can be used to render common input fields, such as textboxes, dropdowns, and checkboxes. The library also includes tags for generating field validation errors and for displaying messages. In addition, the library provides tags for creating complex layouts, such as tabbed forms. The Spring MVC form tag library is an integral part of the Spring Framework and is designed to simplify the process of creating and rendering forms in a web application.