The viewport meta tag tells browsers that a site is responsive, and defines how the content should be displayed on mobile devices.
Without this tag, a mobile browser would display the site at desktop width and scale the whole thing down to the mobile screen size. People would squint at their phones and get headaches. Not a good experience all around.
The viewport meta tag is generated by RapidWeaver based on the theme. It is not a meta tag that you need to create or revise.
SEO-Rx checks whether the meta viewport tag is present. If not, you may see the following alerts:
If you are using a responsive theme and the meta viewport is missing, contact the theme developer.
If you are not using a responsive theme, consider changing to a responsive one, and/or design your pages with responsive stacks.
For more information