What does this number mean?
 

There is a parameter in 4.1.7 called *NIX Server Load Limit

This is what the description says:

"This allows vBulletin to determine the load on the server and processor, and to turn away further users if the load becomes too high. If you do not want to use this option, set it to 0. A typical level would be 5.00 for a reasonable warning level."

What, exactly, does the number "5" mean? Will a higher or lower number turn more users away? How does this work?

It means it will not let new users browse the site till the server load goes below the value you set.

Yes, thank you, I realise that, my question is: What does the number 5 relate to? What is it a measure of? Does it relate to bandwidth, CPU cycles, memory use, what?

How do I know which way to move the number, and by how much, to tune the forum to an average load?

It relates to the server load limit. If you're not on a dedicated server, leave this setting ALONE.

Sorry to point you to the obvious, but if you want to know what exactly server load means - how about start reading up on it here?
http://en.wikipedia.org/wiki/Load_(computing)