Viewport tell the browser , which device are using , means desktop or mobile & set the zoom level of web-pages according to device width. using viewport we can make responsive websites.
name="viewport" → tell the browser how to control the page's dimensions and scaling.
content="width=device-width → sets the width of the page according to the screen of the devices.
content="initial-scale=1.0 → mange the zoom level of the page .