The laser is a block from Buildcraft, that, when used in conjunction with an Assembly Table, can create Buildcraft chipsets and pipe wires. When placed so it is facing an assembly table and powered via an Engine or energy pipes, it will shoot out a laser to the assembly table, powering it.
It has a high power requirement, so while Steam Engines can be used (albeit in large numbers), combustion engines are ideal. If you have only one laser and one combustion engine, do not expect to be crafting loads of materials with your assembly table!
The colour of the laser beam indicates the efficiency and speed at which it works on the Assembly Table (similar to the indication that appears on a Refinery). A red beam is least efficient, while a blue beam is most efficient. There is a bug where the beams appear very dark if the laser is placed at a higher level than the assembly table. One combustion engine burning Fuel or two burning Oil connected via Conductive Pipe will power a laser to maximum efficiency (blue).
If multiple Assembly Tables are in range (4 blocks or less), the Laser will alternate between them.