Basically you point it at some directory containing PHP source code and it generates a set of static HTML pages allowing you to browse the code in your project. The most useful feature is the list of classes, functions (+ methods) and variables it builds. If you need to know all the places in your code a particular function get’s called from, you can find out immediately. Also nice is PHPXref seems to be aware of phpdoc markup  – aware of “doc blocks” at least.[/edit]
Would be nice to have a cross reference for constants and to be able to distinguish global variables quickly, neither of which is currently possible (version 0.4.1) but otherwise, a very useful tool.