It really depends on what system of beliefs you take it from. I believe the most simple idea of it is that the universe eventually gives back what you put out. It might be in this life or another, or in the afterlife.
Depends, Wiccans believe it comes back three times so do something good, somethign three times good comes back to you and if something bad, 3 times bad etc Bhuddists believe its "You reap what you sow" kinda thing.
I believe that its just a way of saying, every cause has a reaction, something negative usually has a negative affect, that then opens more negative options, and closes positive ones.