The JSP API allows to define custom JSP tags that look like HTML or XML tags and a tag library is a set of user-defined tags that implement custom behaviour based on the user's requirement. This directive is used basically for using Custom tags(user defined) in JSP. The general form of this directive is:
Where URI is uniform resource locator, which is used to identify the location of custom tag and tag prefix is a string which can identify the custom tag in the location identified by uri.
As you can see that uri is having the location of custom tag library and prefix is identifying the prefix of custom tag.