Some landlords require a prospective tenant to fill out an application and pay a fee prior to signing a lease. The landlord usually intends to run a check on your credit history or contact references, such as your previous landlord.
Do not pay an application fee unless you are quite sure you want the apartment because it may be difficult to get the money back, even if the landlord rejects you as a tenant. The application fee is viewed as the cost of holding the apartment and not renting it to anyone else while the landlord checks to see if you will be a reliable tenant.