First of all, you may know about meta robots tags.
Meta robots tags are to tell the spider or crawler which page to crawl or index or not.
Follow means: page will be crawled.
No Follow means: page will not be crawled.
Index means: your page show in search Results.
No Index means: your page does not show in search Results.
<meta name="robots" content="index, follow">
This tag tells the web crawler all pages will be indexed and to crawl them.
<meta name="robots" content="follow">
This tag is used to tell the web crawler all pages will be crawled and indexed.
<meta name="robots" content="noindex, follow">
This tag is used to tell the crawler to follow but not index the page in your database.
<meta name="robots" content="index, nofollow">
To tell the web crawler to not crawl the page, but index them.
<meta name="robots" content="noindex, nofollow">
To tell the web crawler to neither index nor crawl.
robots
.) What exactly do you find unclear about it? – Glaze