A reference is a person who can vouch for your character and conduct, whose contact information you include on a rental application so your potential landlord or leasing company can ensure that you're a suitable and reliable renter.
Not all rental applications require references, but whether the application does or does not return a reference, it can't hurt to include a few names and numbers of people who know you well.
Asking a current or former landlord to be a reference for your application is great because your potential landlord will feel comfortable talking with someone who's in the business and knows first hand what kind of tenant you are.
If your rental history is slim or nonexistent, current or former employers are also great options for references, as are family friends or basically anyone who has a close but professional relationship with you.
Generally, all you need to include for your references is the full name and contact information of the person. Contact information should include phone numbers and emails, and an address if your potential landlord requests. Don't forget to ask the person you want to use as a reference for permission to have someone call them.
References are meant to give your potential landlord a sense of who you are so they feel confident renting to you. Think about including them on your rental application even if one isn't required.