Properties are businesses which you can "manage", in the whole game there are about 249 properties for sale, each of those properties has a value between $50.000 and $500.000. You can recognize an available property as a green house marker on the minimap and have "Property:" in its description over icon.
The general way of how a property works is rather simple, every hour the property will have generated the full revenue (this varies between $2.000 and $20.000).
You are able to collect this revenue by walking through the icon of the property.
So if you ever buy a property in the game, make sure that you collect it regularly to make some cash from it.