For the definition of a Configurable Item, please refer to the section Understanding Configurator Products. To add a new Configurable Item to your store, select "Products > Manage Configurable Products > Add New Configurable Item."
You will notice that the page that is displayed has fewer fields than the Add New Product form. The reason is that, unlike Standard Products, Configurable Items are not available for sale as stand-alone products, and therefore many of the settings don't apply.
When you edit a Configurable Item, you will notice the presence of an additional option next to the product's price field. This option has to do with whether or not you want all instances of that Configurable Item to be updated when the price changes. Enable the option if you want all Configurable Products to which you have assigned the selected item to be updated with the new price when the item's price is updated on this page.
There are other ways to update prices within a Configurable Product, including those of Configurable Items that have been assigned to one or more Configurable Products. Please see Updating Configurator Prices.