Questions about implicit waits in Selenium Webdriver

I'm reading the docs on Implicit Waiting with Webdriver but I'm not sure I totally understand.

As I understand it,

driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);

This will put in place a timeout of 10 seconds whenever looking up any element.

What, exactly, does this do?

• Does it mean that when looking up any element, I will wait 10 seconds every time, even if the element is present? Or, when looking up any element, will the driver give the browser a 10-second grace period to load the element before deciding the element missing?

• Since this is applied to every element, am I correct in supposing that this is just executed only once for each instance of Webdriver?