Basically, the meta robots directive is used to specify which pages you want a search engine to index and pass link value through.
So, the directive <meta robots="index, follow"/> on a page tells a crawler that you want that page indexed in search results, and you want the links on that page to pass the link value.
"index, no follow" means "index the page, but don't pass link value from that page"
"no index, follow" means "don't index the page, but pass the link value from that page".
"no index, no follow" means "don't index, don't pass link value"
This directive is mainly an SEO directive and will not affect user experience at all. Pages that are "noindex, nofollow" can still be viewed by users, as it is simply a meta tag in the source.
As to which pages to "index, follow"/"no index, follow", etc. is up to your discretion, but a good rule of thumb is to use it in combination with the link canonical tag to minimize duplicate content pages. For example, wordpress "tag" pages that duplicate the post content from other aggregate pages, are typically good pages to "no index, follow", or e-commerce pages that create duplicate urls from query parameters.
If you haven't already, I'd recommend signing your site up for Google Webmaster Tools, and take a look at the "HTML improvements" section. This will give you a good snapshot of duplicating pages you might want to remove from search results.