A damage deposit or deposit is a sum of money that is paid when one rents an item. It is paid to ensure the owner that it is returned in good condition. It is very common with rented accommodation. There it may also be called a tenancy deposit or in some places a tenancy bond.
The owner of the item (the landlord in the case of accommodation) will take a sum of money from the person(s) renting the item (the tenant). If the item is returned in good condition at the end of the tenancy the owner should return the deposit. If the item is returned with damage the cost of repairing that damage may be charged against the deposit, and part (or none) of the deposit will be returned.