It is more or less the same as writing:
<meta name="robots" content="index, follow">
"all" variant is not actually suggested by Google as an alternative, but it works* nonetheless.
Either command (when placed in the
<head> section of your HTML code) tells search engines to index the page the tag is on, as well as crawl all other pages that are being linked to from that location.*
* That, however, is the default behaviour of search engine spiders — so the necessity of adding the either
"index, follow" on pages has been questioned on multiple ocassions.
N.B. The other commands,
"nofollow", are very useful.