The JSTL <c:out> tag is used to output the value of an expression in a JSP page. This tag is part of the JSTL core tag library, and is typically used to generate dynamic HTML content based on the state of some variable or condition.

The basic syntax for the <c:out> tag is as follows:

<c:out value="${expression}" [default="defaultValue"] [escapeXml="true|false"] />

The value attribute of the <c:out> tag is used to specify the expression that should be evaluated and output to the page. This attribute is required.

The optional default attribute can be used to specify a default value that should be output if the expression is null or undefined. If this attribute is not specified, the output of the tag will be empty in this case.

The optional escapeXml attribute can be used to specify whether or not the output should be XML-escaped to prevent characters such as < and > from being interpreted as HTML tags. If this attribute is set to true (the default), the output will be escaped.

Here's an example that shows how to use the <c:out> tag to output the value of a variable:

<c:set var="message" value="Hello, world!" />

<p>The message is: <c:out value="${message}" /></p>

In this example, the <c:set> tag is used to create a variable called "message" with a value of "Hello, world!". The <c:out> tag is then used to output the value of "message" inside a paragraph element.

The resulting output of this code will be:

<p>The message is: Hello, world!</p>

Note that the <c:out> tag can also be used to apply formatting and localization to the output value, by using the format and bundle attributes, respectively. The <c:out> tag provides a number of other optional attributes for controlling the behavior of the tag, such as scope and var, which allow you to store the result of the expression in a variable.