It is part of the National Missile Defense program.
The system uses a tracking beam to calculate the missile's course and speed. After locking on to the target the weapon class laser will fire a 3 to 5 second burst from a turret located on the nose of the plane, destroying the missile near its launch area.
The laser fired by the weapon is called a Chemical Oxygen Iodine Laser, or COIL, invented at Phillips Lab in 1977.
There is a lot of doubt about whether such a system will in fact prove to be an effective weapon against ballistic missiles, cf. e.g. the APS report on NMD.