The fn:substring function is a JSTL function that returns a substring of a specified length from a given string, starting at a specified index. The function takes three arguments: the input string, the starting index, and the length of the substring.

The syntax of the fn:substring function is as follows:

${fn:substring(string, start, length)}

where string is the input string, start is the starting index (0-based) of the substring, and length is the length of the substring.

For example, the following expression returns a substring of length 3 starting from the 2nd character of the input string "hello":

${fn:substring("hello", 1, 3)}

This expression returns the string "ell".

You can also use variables instead of literals for the arguments. For example, if you have a variable text containing a string, a variable start containing the starting index, and a variable length containing the length of the substring, you can get the substring using the following expression:

${fn:substring(text, start, length)}

This expression returns a substring of length length starting from the start index of the string stored in the text variable.

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" %>