<meta name="robots" content="ALL"> this lins from HTML file. Pls explian me what ias the use of the line.

Answer 1

It is an HTML meta TAG.
They have lots of basically useless but cool uses.
You can add keywords and stuff to them.
Also you know those summaries in search engine results?
That's usually a meta tag.
It is basically the last tag you need to become familiar with when learning html.

Answer 2

it is for searchengine webbots tells them to look at all the content of the page

You can tell bots not to look at the page or to look at bits