A security deposit is a sum of money paid by a tenant to ensure they fulfill the terms of their lease. Remember though – this is a deposit, not a fee. This money should be held in separate bank account and returned to the tenant when the tenant moves out, less any damages that need to be repaired.
The amount you charge is largely up to you, though many states restrict the amount you can charge, so make sure to check to find any local limitations. I typically charge the equivalent of one-month’s rent for a security deposit, though I may charge more than that if the tenant has anything in their background that worries me (which we’ll get to in a short while.)