If you own a residential rental property, you will have repairs and maintenance costs. Once a month you may have a gardener mow the lawn and trim the bushes. Every ten years you may need to replace the roof. If there is a water leak you may need to have your handyman come and fix it. If you are lucky, generally these costs will be manageable. These are all necessary costs to keep the property in a condition that is livable for your tenant. In addition, you could have interest payments on your mortgage and house insurance. We can call these operating costs.
How does your rental property make money? You collect rent from your tenant on a monthly basis for as long as they are living there. Usually tenants sign one year lease, which means if they do not renew after a year, you'll need to find someone else to fill your property. You won't be generating cash flow during the time between tenants, but you will likely still need to maintain the property. In addition, there may be repairs you'll need to make to get it ready for a new tenant that you wouldn't have had to make otherwise. For example, applying new paint to the place will attract more potential tenants.
Remember the goal for investing is putting away money now for more money later. In order to have positive cash flow, you'll want your operating costs to be less than your cash flow in. There are thousands upon thousands of businesses out there that range from simple to understand businesses to complex. The key is to focus on those that you do understand and forget about the ones you don't. In value investing, we call this your circle of competence.