PHP is short name what is officially called "PHP: Hypertext Preprocessor."
Using PHP you can build dynamic website, PHP can connect to various databases, and perhaps the best part of PHP is that its open-source and it can be run on UNIX, Linux and Windows. It can be run virtually on all computing platforms so you can write PHP code on your home computer which has windows and then run the same code on a Unix/Linux Server.
PHP is the most popular scripting language on the web world. It is used to enhance interactive web pages with minimal efforts. With PHP, you can do lots of things like create authentication page, validate forms input, create forums, picture galleries, surveys, and a whole lot more.
PHP is most popular scripting language for server-sided web development. That's because the PHP doesn't get executed on your computer, but on the computer you requested the page from. The results are then handed over to you, and displayed in your browser. Other scripting languages like ASP, Python and Perl. (You don't need to know any of these to make a start on PHP. In fact, these tutorials assume that you have no programming experience at all.)
As a beginner it is easy to start with PHP. The user just has to add a few PHP-tags with e.g. a for-loop in its existing HTML-files and then upload it to the server and see the result or an error message. Dynamic typing and associative arrays makes it also easier to start using PHP.
Compared to most solutions like e.g. Java, PHP doesn't need to be compiled, so it's just to write the script and then upload it to the server and then update the browser.
PHP has (mostly) built-in support for the most popular databases like e.g. MySQL, that means it is easy to start using databases, no additional driver's needs to be installed, just to use the mysql-functions. The easy to use web based admin tool PHPMyAdmin (released 1998) is also important to the PHP's success in combination with MySQL.
PHP became popular early (1995) since it was designed for web programming. Since then the user base has grown and now there is many web-oriented frameworks and libraries available. Some examples are blog-systems and e-shopping-platforms.
Since PHP has existed for long time and works good on both Linux and Windows, and many web servers have support for it. There is no problem to find hosting with PHP pre-installed.