A security deposit is money a tenant pays at the start of a lease that the landlord holds — it is not income — as protection against unpaid rent, damage beyond ordinary wear and tear, or other lease breaches. At move-out the landlord returns it, minus any documented, allowable deductions.
The details are heavily regulated and vary widely: how much you can collect, whether it must sit in a separate or interest-bearing account, what you may deduct, and how quickly you must return it with an itemized statement. Rules vary by state and locality — check your local law, and treat this as general education, not legal advice.
This is general educational information, not legal or tax advice. Rules vary by state and locality and change over time — check your local law and confirm specifics with a qualified professional.