A diamond's cut actually refers to two things. First of all, the diamond's cut refers to it's shape, it's outline. Take the heart shape for instance. That part’s simple.
Cut also refers to the diamond’s proportions – and this is extremely important when determining the value of the diamond. When a diamond is cut too deeply, light comes in through the top, leaks out at the bottom and you get a diamond with a very dark center. It's not very attractive.
When a diamond is cut too shallow, the sides are very open, light again comes into the top and leaks out through the bottom and you get what's called a “fish eye” which is a dark circle surrounded by a white ring.
However, when a diamond is cut properly, the light goes in through the top, bounces around from facet to facet, it shoots back out at you and that's when you get a diamond that you fall in love with, just dazzles you.