A no index tag tells Google or other search engines not to index a specific page. If a page is not indexed, it will not be found via search. There are some cases where no index tags should be used such as:
Email Promotion- some promotion pages are meant to be communicated only to segmented customers
Employee Only Pages - some links may be meant for internal source
Goal Completion Pages - an example of this is thank you pages that come after completing a purchase online. If this page were to be indexed then it would confuse visitors and distort data, which is used for analytical purposes, as no purchase has been made
Duplicate Content- if a page is a duplicate of another page
How to Create a No Index page
This is what a No Index Code Looks Like:
<META NAME="ROBOTS" CONTENT="NOINDEX,FOLLOW">
This code tells robots to not index the specific page but that it is still related to the pages that are linked to it.