What does this code mean and what does it do?
<META NAME="robots" CONTENT="index all, follow all">

Can anyone give me a concise description?

In other words, not just > It tells "robots" to run through your site, or Why do you want to know, or it depends on "---", or it only works on "---", etc.

Thanks in advance for helpful, clear, pertinent, concise responses

It means nothing at all as you have it, because it is coded incorrectly.

http://www.robotstxt.org/wc/meta-user.html

the correct form of

<meta name="robots" content="index,follow">

tells compliant bots nothing at all because "index , follow" is the default and the robots meta is an exclusion protocol. So it is used to stop compliant bots from performing an action.