As you may already know, the bedrock or better still, the OS of a webpage is HTML. So when a someone wants to visits a page in a website through the web browser, the browser request for the HTML resource of that page from the web server, it then parses the HTML content/file returned by the web server and display it to the user.
This means that two connection type -HTTPS and HTTP- was used before the complete content of the page is displayed.
The Main Point
When all or some of the content of a web page is not loaded using the same HTTPS connection which was used to initial the connection, then that page is considered to be a mixed content page, because that single page was loaded with differing connection types -HTTPS and HTTP.