PHP is a widely-used open source general-purpose scripting language that is especially suited for web development and can be embedded into HTML. PHP is known as a server-side language. 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.
The best things in using PHP are that it is extremely simple for a newcomer, but offers many advanced features for a professional programmer. Don't be afraid reading the long list of PHP's features. You can jump in, in a short time, and start writing simple scripts in a few hours.