use a Meta tag if you have the odd page in indexed folders that you want skipping, but generally, I'd recommend you most of your non-indexed content in one or more folders and use robots.txt to skip the lot.
Robots.txt is a text file that lists webpages which contain instructions for search engines robots. The file lists webpages that are allowed and disallowed from search engine crawling.
Both robots.txt and meta tags are important On-Page SEO techniques. Robots.txt is for web crawlers to stop it from indexing it and meta tags are used in HTML codes for SEO purpose.