What is the function of "noIndex, follow" attributes in Metatags?

[Venu]

I know noindex, nofollow don't display website in the Google Search results.
Google bot not "index" the content in the database.
Google bot index the webpage, but not not give positive rating to the webpages.
but another commend "noindex, follow", how is this possible?
Without indexing the content, how to give positive rating to the webpage?
Please explain depth of the "noindex, follow" attributes?
Without indexing a webpage how to give link juice?
It means crawling and indexing both are different.?
It means after crawling give link juice without indexing (display on the Search results) a website?