You have already encountered some function calls. Function calls perform some computation and return a result; they can accept arguments, which are inputs to that computation, in some cases.
If there are no arguments, you write the function call with a pair of empty brackets, like so:
If there is just one argument, you place it inside the brackets:
function-name ( argument expression )
If there are two or more arguments, you write them in a comma-separated list:
function-name ( argument expression , argument expression , ... )
XPath itself provides a broad selection of useful functions, and extra ones can be provided by the application using XPath; for example, XSLT adds the “document” function and a few ...