The <x:set> tag in JSTL (JavaServer Pages Standard Tag Library) is used to set the value of a variable to the result of an XPath expression. Here's an example of how to use it:

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/xml" prefix="x" %>

<x:parse var="myXml">
  <root>
    <item id="1">Item 1</item>
    <item id="2">Item 2</item>
    <item id="3">Item 3</item>
  </root>
</x:parse>

<x:set var="firstItem" select="$myXml/root/item[1]"/>
<x:set var="itemId" select="$firstItem/@id"/>
<x:set var="itemName" select="$firstItem/text()"/>

<p>Item ID: <x:out select="$itemId"/></p>
<p>Item Name: <x:out select="$itemName"/></p>

In this example, we use the <x:parse> tag to parse an XML document into an object named myXml. We then use the <x:set> tag to set the value of three variables: firstItem, itemId, and itemName.

The select attribute of each <x:set> tag is used to specify an XPath expression that selects a value from the XML document. In the first <x:set> tag, we select the first item element from the myXml object. In the second <x:set> tag, we select the value of the id attribute of the firstItem variable. In the third <x:set> tag, we select the text content of the firstItem variable.

Finally, we use the <x:out> tag to output the values of the itemId and itemName variables.

Note that the <x:set> tag can also be used to set the value of a variable to a literal string, like this:

<x:set var="myVar" value="Hello, world!"/>

In this case, the value attribute is used to specify the literal string to be assigned to the variable.