Freebie is the rule, Sans and Papyrus created. You have three freebies. When you're stuck, show with your fingers how many freebies do you still have, and one of the skeleton brothers will help you. Papyrus mistaken it for frisbees for many times. Sans is kinda annoyed with it.
Snowdin Forest Edit
On the first puzzle, Papyrus explained what freebie even is. Aliza tried to use it when she's stuck, but her hands sinked in, so she couldn't use it. So she died. On the second try, Aliza realized there is no way the puzzle could be done by her. So she used her first freebie, and Papyrus had to help her.