Responsive design
A website layout that adapts itself to the width of the screen, from a phone to a wide monitor.
Glossary · Websites & e-commerce
Responsive design means the same website, the same address and the same content are shown on every device, and only the layout changes: columns stack, the menu collapses, type and spacing adjust.
It is not a separate mobile version of the site. Separate versions mean two sets of content and two addresses to maintain, which search engines may treat as duplicates.
On a site that responds well, a phone is comfortable not just for reading but for doing things: buttons are large enough, forms are short, and images are sent at the size the screen needs. Google evaluates websites by their mobile version, so it is not the secondary one.