a web site can have a mix of PHP and HTML pages, this is what was meant by sayin that eCart can be integrated into an HTML site.
eCart will place server side code on your pages.
The server side PHP code cannot be added to an HTML page. It can only be added to a dynamic PHP page.
You will need to create new pages to add the eCart functionality to and give them the file extesnion .php.
If you wanted to add the eCart functionality to the HTML pages you have already created, they will need to be renamed with the PHP extension.
This will likely break links between the pages until you edit the linking urls to use the php extension as well.