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:split function, which is used to split a string into an array of substrings based on a delimiter.

The fn:split function takes two arguments:

1. The string to split
2. The delimiter to split the string on

The function returns an array of substrings obtained by splitting the input string at every occurrence of the delimiter. The syntax of the function is as follows:

${fn:split(string, delimiter)}

where string is the string to split and delimiter is the delimiter to split the string on.

For example, to split the string "foo,bar,baz" into an array of substrings based on the comma delimiter, you would use the following expression:

${fn:split('foo,bar,baz', ',')}

This expression returns an array of strings: ["foo", "bar", "baz"], where each element in the array is a substring obtained by splitting the input string at the comma delimiter.

You can also use variables instead of literals for the arguments. For example, if you have a variable text containing a string, you can split it into an array of substrings using the following expression:

${fn:split(text, ',')}

This expression returns an array of substrings obtained by splitting the string stored in the text variable at every occurrence of the comma 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.