Welcome to English in a Minute!  
When you throw a boomerang, it comes right back to you. That might help you understand this saying: What goes around comes around. 

D: Anna, I brought in muffins. But Bob from accounting ate them all.
A: Well, that explains it. 
D: Explains what? 
A: Bob is in the hospital with a muffin stuck in his mouth.  
D: Well, what goes around comes around. Bob’s going to think twice before eating muffins meant for everyone.  

 When we say, “what goes around comes around,” we mean that actions have consequences. So, if you behave in a bad or dishonest way, something bad or unpleasant might happen to you later.