Speed Settings let you control how this page is cached.
Caching is the concept of pre-building parts of your page on the server so they can be delivered faster. Caching is not a guaranteed performance benefit. In fact, if you have a server with a slow/busy hard drive turning on caching may SLOW your site down.
To try out caching, first turn on full page caching in the Dashboard by going to the Cache & Speed Settings page. (Type "Cache" in intelligent search!)
Once caching is enabled, you will be able to configure it on a page by page basis as needed.
You may want to turn off caching for a page that has a lot of dynamic content like forums.
The length of time you cache pages should generally be as LONG as possible. This time setting is how often concrete5 will rebuild these pages, so think hours not minutes.
There are other caching layers you can turn on for concrete5 as well, including APC and MemCache. Check out the How-To section for more Caching information.