Premium Domains are already registered domains. They are owned by someone and are for sale.
Premium Domains have usually shorth-length names are easy to memorize and are valued much higher thatn all others.
They may cost a small amount or even reasonably large amounts.
Some other terms used, are:
Secondary Market Domains
Most of these are ".com" , ".net", ".org", ".biz" domains.
Some of them may already have serious ready/active/traffic coming in,
and even remain at a high rank in Website Ranking sites, suh as Alexa.
If you are lookiing for an established Domain name that is already known and has a lot of traffic already coming in,
then this is a good option. In case your niche is the same or very relative to a already existing Premium Domain this can give you a "fly start"
than taking all the effort from the very beginning.