about viewport meta tag

Tells the browser that this webpage is optimized for mobile, changes the logical window size used when displaying a page on mobile browsers.

Include width=device-width to match the screen's width in device-independent pixels, and please ensure your page is accessible by not disabling user scaling.

The presence of this tag indicates to Google that the page is mobile-friendly, this is very important as Google gives prioriy to this kind of pages in its SERPSs. Don't forget to build the page using responsive design to make it really mobile-friendly.

Without the vieport meta tag, the First Input Delay (FID) could get lower scores.

content examples

The advised parameters for a mobile-friendly page is the following:
<meta name="viewport" content="width=device-width,initial-scale=1">

The exception for not allowing page scaling is when a page is a PWA (Progressive Web App) as they mimic native apps behaviour.
<meta name="viewport" content="width=device-width, user-scalable=no" />