The Simple theme has been built using responsive styling. This means when you reduce the width of your browser window, the site will adapt to the screen size, a technique made possible using HTML5.
Responsive web design is an approach aimed at crafting sites to provide an optimal viewing experience - easy reading and navigation with minimal resizing, panning, and scrolling - across a wide range of devices (from mobile phones to desktop computer monitors).
This behaviour is provided by the default with the theme. Alternatively, you can also view this page on your favourite smartphone to see how the theme adjusts to a specific device. The menus and other elements of the page will change to be mobile or tablet friendly.