It could be your vacation home that you rent out for part of the year. It could be a multi-unit apartment building. Income Income is the simple thing about real estate. You collect rent. The IRS doesn't worry about rent too much.
People pay rent regularly.
It's easy to trace. The IRS likes you to report these as income when you get them. If that's your position, keep the security deposits in a separate account. Don't mix it with your own money or the IRS will say that you treated the money as your own. Of course, if it's income when you receive it, then it's a deduction when you pay it back.