It's when someone uses the holidays to make you think your relationship is more serious than it really is. For example, they might get you a romantic Christmas gift, or let you meet their family. But once the holidays are over, things cool down fast.
Sometimes it's malicious, but it can also happen by accident if you just get carried away with the romance of the holidays. It sort of goes hand in hand with "cuffing season," when people look for someone to hook up with until the weather gets nice again. Then they move on.