Описание тега initial-scale

Initial-scale is a property of the viewport meta tag which is used to set the initial "zoom level" on a mobile browser.

The initial-scale property controls the zoom level when the page is first loaded. The maximum-scale, minimum-scale, and user-scalable properties control how users are allowed to zoom the page in or out.

Further information on viewport meta tags can be found here: https://developer.mozilla.org/en-US/docs/Mobile/Viewport_meta_tag