The <fmt:bundle> tag in JSTL is used to load resource bundles that contain messages, labels, and other localizable data. The resource bundles are typically stored in properties files.

The basic syntax for the <fmt:bundle> tag is as follows:

<fmt:bundle basename="bundleName" var="bundleVarName" />

The basename attribute specifies the name of the properties file that contains the resource bundle, and the var attribute specifies the name of the variable that the resource bundle will be stored in. The basename attribute can be an absolute or relative path to the properties file.

Here's an example that shows how to use the <fmt:bundle> tag to load a resource bundle and output a message from it:

<fmt:bundle basename="messages" var="msg">
  <fmt:message key="welcome.message" />
</fmt:bundle>

In this example, the basename attribute specifies the name of the properties file that contains the welcome.message key. The <fmt:message> tag is used to output the message corresponding to the welcome.message key. The message is retrieved from the resource bundle and stored in the msg variable.

Here's an example of a properties file that contains the resource bundle:

# messages.properties
welcome.message=Welcome to our site!

When the JSP page is rendered, the output will be:

Welcome to our site!

The <fmt:bundle> tag provides a convenient way to load and use resource bundles in JSP pages, and makes it easy to support multiple languages and locales in your application.