Good thread on the link Biggedon posted with lots of good thoughts, make sure you read it.
IMO, on one hand, like many have said here, "premium", when used honestly means the name is a lot more valuable than other names you might see. So, for a dictionary word, it might be a real word found in a normal dictionary, not an obscure word, or one with -ly, -ing, -ed, or other tenses. Books vs. Booked vs. Bookly. Sure, the last one might be in someone's word list, but the first one is a premium in anyone's mind.
For LLL/LLLL/LLLLL type names, where the combination of letters don't spell anything, premium is sometimes used to refer to the individual letters in the name. Some letters have been labeled as "premium" by domainers because they are more popular in the English language. So, when hyping these types of names, you'll see people refer to the number of "premium" letters in the name. Of course, once you look at the popularity of letters from a non-English language, then the label premium can change.
Lastly, like most people said in the other thread, "premium" is the most over used word in domain selling. Sellers use the word so often, that it has lost any value to describe a name in a sale.
