In HTML5 feature is for web developer or designer to take control the viewport, using this meta tag.
It is used for view the web pages as viewer requested specifications i.e width and height of based on screen resolutions automatically it will adjust the web page.
As a meta tag where ever this feature is require you need to use this as metadata.
Will see an example with its usage
In HTML base tag is used to specify base URL and base target for all relative URLs in a page.