Your windshield is two panes of glass, bonded by a sheet of laminate. When a rock or other object damages your windshield, some of the glass will disintegrate and separate from the laminate. This results in an area of empty space between the laminate and the surface of the glass. Energy can travel through this empty space and cause the rock chip to turn into a long crack. To repair your windshield, we inject this empty space with an acid based resin. Once the damage is completely filled, we cure the resin with UV light. This process will prevent energy from traveling through the empty space.
Windshield repair is a great solution when conditions are right. The damage, in general, must not be bigger than the size of a quarter. When rock chips are bigger than this, they usually will not fill completely. If the rock chip is not filled completely, then energy can travel through the unfilled space and turn rock chips into long cracks.
The repair process does not cause the damage to disappear. The rock chip will certainly be less visible. And I will confidently say that no one does better looking repairs than the Windshield Doctor.