What is it?
A general purpose scripting language.
"PHP is a server-side scripting language designed primarily for web development but also used as a general-purpose programming language. PHP originally stood for Personal Home Page, but it now stands for the recursive acronym PHP: Hypertext Preprocessor". Source: Wikipedia.
PHP powers your website.
"Each release branch of PHP is fully supported for two years from its initial stable release. During this period, bugs and security issues that have been reported are fixed and are released in regular point releases.
After this two year period of active support, each branch is then supported for an additional year for critical security issues only."
There are a number of versions in common use but only the more recent versions are supported by developers.
Check whether latest PHP release is a Security Fix
It is not immediately obvious whether or not the latest release of PHP is a Security Fix when you read the Change Log for any given release of PHP.
One factor to consider when upgrading PHP is that some 3rd party Joomla! extensions require specific PHP Modules to be installed for them to work.
When you upgrade PHP and see a warning next time you visit your website, then you will most likely need to check your server's PHP settings.
PHP settings can usually be adjusted in your Hosting Control Panel.
It may however be necessary to contact your hosting company's Technical Support.