Being a landlord. There are many trades for which you need to know something about the house structure (civil engineering), electric (electrical engineering), plumbing (civil and mechanical engineering), HVAC (mechanical engineering), etc. This will help you to maintain your rental properties and deal with contractors so that no contractors can take advantage of you. There are plenty good and honor contractors http://bbs.wenxuecity.com/tzlc/1187631.html, but you need to know something to protect yourself.
You need to know something about finance and investment which will help you to gauge the economic environment in your area, valuate/buy/sell rental properties, understand the potential renters credit history, etc.
The most important is human psychology, which include communication and psychology. How to select a potential renter, how to deal with renters, how to solve any issues in the rental properties raised by your renters.