what does viewport do in html code example

Example 1: viewport meta tag

<meta name="viewport" content="width=device-width, initial-scale=1">

Example 2: how to set viewport in html

<meta name="viewport" content="width=device-width, initial-scale=1.0">

Example 3: css viewport

<meta name="viewport" content="width=device-width, initial-scale=0.9, user-scalable=no">