What is the 'Itemid' in a Joomla URL?
If you examine the non-SEF url of a typical page on your Joomla site you'll see the ItemId parameter, here's a typical example for one of the pages on our site:
ItemId refers to the ID of the menu link associated with this article, and searching your menu items for this ID will show that actual menu link.
Why is Itemid Important in Joomla?
As you probably know, Joomla let's you customise layout and module assignment based on menu items, and the way it tracks this is by using Itemid, so that's why it's there and why it's important.