The JSTL (JavaServer Pages Standard Tag Library) fmt tag library provides a set of tags for formatting and internationalizing data in JSP (JavaServer Pages) pages. One of the most commonly used tags in the fmt tag library is the <fmt:message> tag, which is used for internationalizing text messages.

The <fmt:message> tag retrieves a text message from a resource bundle and formats it according to the specified parameters. The syntax of the tag is as follows:

<fmt:message key="messageKey" [var="varName"] [bundle="bundleName"] [varargs="true|false"] />

where messageKey is the key of the message to be retrieved, varName is the name of the variable to store the formatted message, bundleName is the name of the resource bundle to use, and varargs is a flag that indicates whether the message supports variable arguments.

For example, to retrieve the message with key "hello.world" from the default resource bundle and display it on the page, you would use the following tag:

<fmt:message key="hello.world" />

If the default resource bundle contains the message "Hello, World!", this tag would output that message on the page.

You can also use the <fmt:message> tag to format messages with variable arguments. For example, if the message with key "welcome.user" in the resource bundle contains the placeholders "{0}" and "{1}" for a user's first and last names, respectively, you can use the following tag to format the message with the names:

<fmt:message key="welcome.user" var="welcomeMessage">
  <fmt:param value="${user.firstName}" />
  <fmt:param value="${user.lastName}" />
</fmt:message>

<p>${welcomeMessage}</p>

This tag uses the <fmt:param> subtag to pass the values of the user's first and last names as arguments to the message. The formatted message is stored in the variable welcomeMessage, which is then displayed on the page.

Note that the fmt tag library must be imported at the beginning of the JSP page using the following tag:

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

This imports the fmt tag library and assigns it a prefix of fmt.