Overclocking is raising the clock speed or instructions per cycle so a GPU or CPU can get more done in the same amount or less time.
While doing this however, you need to raise the voltage of the GPU to keep up with the faster clocks, and you can only go so high with overclocks. It really comes down to luck, or the "silicon lottery" as many call it.
Think of your GPU as a runner in a marathon. Think of his pace as the clock-speed. Think of voltage as an energy drink.
To make him go faster, he needs more energy. Making him go faster without more energy will probably make your system crash.
too much energy drink, however, and you can potentially damage your runner (GPU). Same with heat, the more voltage a card has, the more heat it creates.
I hope you enjoyed this little analogy, and good luck overclocking!