Responsive web design
Responsive web design uses flexible grids, fluid images and usually CSS media queries so one website adjusts itself to a phone, a tablet or a desktop screen.
The same page rearranges itself to fit the screen it is on, instead of needing a separate site for each device.
Why it matters for a small business
Many customers will look you up on a phone. Open your site on yours and check you can read and tap everything.
Source: MDN, 'Responsive web design'