A "slip" is like a form, but it's usually smaller than a full sheet of paper. Some examples of "slips" are:

• permissions slips (for school field trips, etc.)
• deposit slips
• wire transfer slips
• a pink slip (which is a piece of paper telling someone that they've been fired or laid off)

"Deposit slips" are pieces of paper that you have to fill out when you're depositing money into a bank account. You write your name, the amount of money you're depositing, your account number, and so on.