Answer 1

That tag makes sure that your page does not stay in the cache of your visitor's computer. It is useful if the content of the page changes regularly, since in those cases it does not make sense to store everything in the cache (although there are exceptions if your site is very heavy, but most consist of other methods).

You can also use a date there. In any case, here I leave you much more information:

https://www.w3.org/Protocols/rfc2068/rfc2068