The number reflects how many items they expect to be able to identify in that category. They bid until someone reaches 7 or passes. Then the main game board appears, showing the category name and 12 items that may be members of that category - 7 of them are, but the other 5 are not. Click enough correct items to meet your bid. Watch for errors - your opponent gets the points.
Registration gives access to many more categories than are in the download.