Responsive
A responsive page changes its layout to fit any screen, from a small phone to a wide computer.
In a simple example
On a phone the menu folds into one button and the pictures stack in one column. On a computer they sit side by side.
Why it matters to you
Most visitors use a phone, so a page that is not responsive loses many of them.
Do not confuse with
Responsive is not the same as a separate phone app. It is one site that fits every screen.