Yea it just means four letter domain or LLNN two letters and two numbers. Three and four letter domains are worth a lot so they are always popular. I just got a LLLLL.com and was excited.
Why are they worth alot... I cant see the point in wasting my time trying to build a brand/image for domain like "a8n.com" "s88g".. Stupid stuff like that.. Explain to me how they are valuable..
The smaller the name, the easier it is to remember for people. Short names were the first taken in the domain names. It is rare to find any three letter so it makes it more valuable.