The JSTL (JavaServer Pages Standard Tag Library) fn tag library provides a set of functions for manipulating strings, collections, and other objects in JSP (JavaServer Pages) pages. One of the functions in the fn tag library is the fn:join function, which is used to concatenate the elements of a collection into a string using a separator.

The fn:join function takes two arguments: the collection to concatenate, and the separator to use between the elements. The function returns a string containing the concatenated elements of the collection separated by the separator. The syntax of the function is as follows:

${fn:join(collection, separator)}

where collection is the collection to concatenate, and separator is the separator to use between the elements.

For example, to concatenate the elements of the collection [1, 2, 3] into a string separated by commas, you would use the following expression:

${fn:join([1, 2, 3], ',')}

This expression returns the string "1,2,3", which is the elements of the collection separated by commas.

You can also use variables instead of literals for the collection and separator arguments. For example, if you have a variable numbers containing a collection of numbers, and a variable delimiter containing a separator, you can concatenate the elements of the collection into a string using the following expression:

${fn:join(numbers, delimiter)}

This expression returns a string containing the elements of the collection separated by the separator in the variable delimiter.

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

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

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