Mobile Friendly Responsive Web Design
In the past the only way people looked at the web was on a desktop or laptop computer. Websites were designed to work with as many Browser and Operating System combinations as possible. Most websites were viewed at roughly the same size.
Now we have tablets and smart phones. Viewing a website designed for a desktop computer on a small smart phone can be a pain; expanding a site to read the text, swiping around to get to a section of the page and trying to click on minuscule buttons. To deal with this problem, web developers started making a separate site (or several) for smart phones. That way the smart phone user could navigate through the site, usually an abridged version of the main one, more easily.
Technology marches on and now the trend is to make one version of the website that changes the layout to accomodate the device it's being viewed on. It's a completely different way of designing and is very complex. This is called “Responsive Design”.
To see this in action you can look at this website. If you make the browser window smaller or larger, you will see the layout change.
Example of responsive design.