How does the Cap Rate Work

[Daniel P Willis]

So I have a single family home and a trailer that I rent out and my wife and I are looking at purchasing a triplex or quad. How does the Cap Rate work and at what time does that become relevant? I always try to learn as much as I can before making an investment and most of the forms that i've seen just say that cap Rate isn't something you have to worry about until its a commercial property. Why is this, why doesn't it matter and how does it work? 

     Any help is much appreciated, I would also like to know your experience with using Cap Rate and how it helped you in your investment, if its not to much trouble. 

   Thank you.