PHP is a general-purpose scripting language geared towards web development. PHP code is usually processed on a web server by a PHP interpreter implemented as a module, a daemon or as a Common Gateway Interface (CGI) executable.
By default Nginx is not capable of processing dynamic web pages with PHP so it requires the use of specific additional plugins for this purpose. One of these plugins is FPM (FastCGI Process Manager). FPM is an alternative PHP FastCGI implementation with some additional features (mostly) useful for heavy-loaded sites. It is the preferred method of processing PHP pages with Nginx and is faster than traditional CGI based methods.