According to proponents of this law, thoughts have an energy that attracts like energy. In order to control this energy, proponents state that people must practice four things: 
Know what you want.
Ask the universe for it.
Feel and behave as if the object of your desire is on its way.
Be open to receiving it.
Thinking of what one does not have, they say, manifests itself in not having, while if one abides by these principles, and avoids "negative" thoughts, the Universe will manifest a person's desires. 
Are you a believer, or non? I am a believer, although I admit freely that once I began to subscribe to this line of thought, my focus on the positive (and my purposeful habit of disregarding or refusing to "absorb" the negative) creates a breeding ground for synchronicity, causing me to be completely unable to truly, objectively, decide if it has caused more good to come into my life, or if it has just caused my perception and focus to shift.
Not that it matters, really - either way, life becomes more positive. Clouds start having silver linings, which I continue to focus on. And so the circle goes on.