I think its getting a card up to a point where it is almost max exp / level. Then they sell it on the market at a higher price because it means you'll only need 1 or so wins to get it to max level and gain alot of battle points or w/e. They are usually 5* star cards, so a pre-full card would be 4 stars and almost to its fifth.
Prefulling is getting a card to just one game away from max leveling. For example, getting a 4* to 2995 XP, when the maximum is 3000. Cards like these are sold for more than market price, and people who buy them use them to level up faster, either by using their own reserve XP or by leveling them in Lost Warehouse. These are the players who you see get 20k+ BP a day without any effort.