Responsive design
aka: mobile
The Quick Version
One site that works on a phone, a tablet and a 32-inch monitor.
How it actually works
Responsive design uses flexible layouts and breakpoints so components rearrange rather than shrink. It is tested by resizing, not by assuming.
Why does it matter?
More than half of your visitors will arrive on a phone. Probably more.
Example
A three-column grid collapsing to one column below 768px.