Help with understanding Cap Rate in it's simplest form

[David Jenkins]

Hi -- Can someone please explain how I'd be able to figure out a cap rate on a calculator as if I were a 5 year old?

I've seen several other investors try to explain this and it still looks like Chinese to me (and I don't speak Chinese). 

I'm a realtor / investor who actually owns several smaller apartment buildings / units and have always relied on a simple napkin analysis of: How much cash flow I wanted to earn after downpayment. I've never used the traditional formulas of cap rate / GRM / ect -- However, I want to purchasing much larger buildings (20+ units) and think having a basic idea of the fundamentals is probably necessary. Thanks!!