PHP is a scripting language for developing webpages with dynamic content. HTML by itself can only handle text, images, and some formatting. On the other hand, if you want to make your the content on your site a little more interactive, PHP may be the right tool to use.
A common use of PHP is creating a content management system. A content management system allows you to separate content on a website from the general appearance or theme of the website. It is inefficient to consistently have to create and edit a new webpage by hand every time you want to add content to a website. A content management system allows you to create your content and then easily have it inserted into a pre-made layout. This means you can edit the appearance of your site without having to change each individual webpage. PHP can be used to create a web-forum or other sorts of subscription based sites that must handle user accounts. Possibly the most profitable use of PHP is the fact that it can be used to create an online web-store similar in order to sell your products online. Simply using HTML will not be enough for this.