What is PHP? I could just have you visit php.net and find out for yourself, but although you would find out a lot of information about PHP you probably wouldn't really understand its use/purpose.
When you create a website using just HTML and CSS, it's "static". This means that no matter who accesses each page, all of the pages will look the same.
The reason for this is that with static web pages, the server simply regurgitates the same page (code) to each viewer. In most cases, you are going to want something better: a dynamic web page.