SPF

   The Sender Policy Framework (SPF) is an email-authentication technique which is used to prevent spammers from sending messages on behalf of your domain. With SPF an organisation can publish authorized mail servers.

Viewport - initial-scale

   The viewport is the user`s visible area of a web page. The viewport varies with the device, and will be smaller on a mobile phone than on a computer screen. Before tablets and mobile phones, web pages were designed only for computer screens, and it was common for web pages to have a static design and a fixed size.

Disable Telephone link in HTML on Mobile Devices

   Telephone links are a thing. Like an anchor link you tap to (probably) go to another page, these are links you tap to call a number on a phone-capable device.

No Cache Content

   Forces caches to submit the request to the origin server for validation before releasing a cached copy.

NO Store HTTP Cache-Control

   The cache should not store anything about the client request or server response.