A "deposit" is some money that you pay at first when you're reserving something. You might need to pay a "deposit" in situations like:

• when renting a car
• when signing up for classes
• when renting an apartment

You can express the amount of a deposit like this:

a $400 deposit 

a 10% deposit

Sometimes a deposit is "non-refundable". This means that you can't get the deposit back after you've paid it.