The title shows at the top of the page. The description is invisible to the page, but appears in search engines. Keywords are invisible to both, but help "define" the page.

Here's an example:

Title: Joe's Pet Shop
Description: Joe's Pet Shop services the greater Northwest region, providing leashes, kibble and all the other goodies your furry friends want most!
Keywords: pet store, pet supplies, pet food, northwest, kibble, furry

I don't exactly know how this interacts with Concrete's search feature. I would guess that all of these would be indexed, but the title probably has the strongest weight, followed by either the description or keywords ... not sure. I also hear that the search has been improved in Concrete 5.4, which is releasing soon.

the Keywords and Meta tags are used by search engines to better classify your site and its content, like if your site is about Mac but you right a page about windows you would add keywords like windows, pc etc