What exactly does "Add No-cache headers" do?

[Gutspiller]

what does "Add No-cache headers" do when it's turned on it admin options?

What does it not do when it's off?

[Xenon]

it forces the browser everytime download the page from the searver instead of reading out from it's local cache.

if you turn it of, you would get cached browsing results, which are not as actual as the noncached :)