Explanation's not needed because, in general, it's simple. You got modded down because what you said was interpreted as contributing nothing. How can you say something and contribute nothing? There's many ways:
- Trolling (examples: "you're clearly a mac fanboy", "you want fries with that?")
- Be trite or redundant (examples: "lol", "i agree", "+1 interesting", "upmod parent")
- Personal attacks (examples: "I bet if you were him you'd say 'go away customer, I only write my software in Rails/LISP/Haskell'")
- Poor attitude (examples: "shut the f*ck up", "that's dumb")
- Being incoherent, in jest or in seriousness (examples: "mahler called, he wants his symphony back")
- Rambling on without saying anything (examples: "then i went to see my dentist, but that's a whole another story...")
- And, last but not least, disagreement. Theoretically, you will not be downmodded for saying something disagreeable. In practice, however, downmodding on disagreement does happen. Some misuse is to be expected, and all we can do is live with that.