The GRAF (GRAvitational Focus) shield was designed by the former University of Chicago instructor Professor Algernon for the Neosapien Empire. Before the start of the Pirate Clans War, Algernon had offered the shield to the Homeworlds Congress. However, the Congress had refused the offer since there had been a 50 year peace.
The GRAF shield consists of a dome or series of domes that can be powered up and its energy directed at a target. Because it is an emerging technology that cannot be explained by traditional fields of physics, and it is one with military applications, little information regarding it has been made public knowledge.
From appearances, the GRAF shield seems to increase the strength of the force of gravity within its target range. Target range is between high orbit and the upper atmosphere of a planet. Spaceships in its field begin collapsing in upon themselves, and can easily be reduced to one-tenth their normal size if they don't explode first.
Vessels that use a Cloaking Device (a capability Pirate ships have) are unaffected by the field produced by a GRAF shield, as the shield's targeting sensors act like light waves. Algernon summarizes the limitation in as: "they can't hit what they can't see".
At the current level of technology the GRAF shield is used as a defensive weapon. Installations must be built on a planet or moon, and the shield requires a great deal of energy, more than a spaceship could produce.