<meta name="viewport" content="width=device-width, initial-scale=1.0">
This tells the browser to make its viewport match the particular device's default viewport.
- For iOS devices (iPhone), that's 375 CSS pixels wide (414 for a "plus" model).
- For Android devices (Samsung, LG, Google), that's in the range of 360-480 CSS pixels wide.
- For various tablets, that's in the range of 600-1024 CSS pixels wide.
Interesting Comparison
Load this page both in a mobile device and in Chrome on a traditional screen. On the traditional screen, open the Chrome developer tools (View->Developer->Developer Tools), and click on the little cell phone looking icon (Toggle Devices Toolbar). That should put this page in a small window that you can resize to simulate different device widths. If you resize that to simulate your device width, this page should look about the same as it does on your mobile device. (Or you can use this to figure out about how wide your device's default CSS pixel width is.)