Fast Search for SharePoint (and SharePoint 2013) has a feature called Managed Properties. These are, in short, operators that trigger search to key in on a specific property/metadata (e.g., a field in a list or library). If you read my previous post on search; Enterprise Search - Boolean and other advanced search techniques; you'll see that there are many out-of-the-box managed properties that are mapped. A couple of examples are "Filename:" and "Author:". To quote the official documentation, a SharePoint farm administrator or search service application administrator "can enhance the end-user search experience by mapping crawled properties to managed properties. Crawled properties are metadata (such as author, title, or subject) that are extracted from documents during crawls. Managed properties can appear in refined searches and help users perform more successful queries."
As a rudimentary example of how one would use a custom (not out-of-the-box) managed property, let's say you have a SharePoint site, and on that site you have a list containing weapon system parts. In this list, you have a column called Weapon System that is populated with data indicating what weapon system the part fits. You could create a view that filters to just that weapon system, but if you wanted to perform a search that returns all parts that go to a particular weapon system, we could create a managed property named, for example, PSWeaponSystem, and mapped that managed property to the [Weapon System] column in your list (via its crawled property). this would allow you to go to the search center and enter in PSWeaponSystem:"Name of the weapon system". Since the managed property is mapped specifically to a column on your list, it will not return extraneous/unrelated records, and would only return records from your list that match data in that column (Weapon System in this example). At the risk of opening a Pandora's Box, SharePoint farm admins can pass this information along to site admins allowing them to request a managed property. Site admins should clearly define their business needs and what they want to accomplish with search. Each managed property would need to be evaluated by a farm admin since it can create a burden on the search system, but managed properties certainly can make make SharePoint search more useful and productive if used correctly.