Blutz Waves are an energy source emitted by large celestial bodies such as a full-moon or the Earth (in Goku's case during Dragon Ball GT) when it reflects rays of sunlight. They are alternatively called Bruits Rays, Fruits Rays, or Green-spectrum Radiations (in the English manga).
When a Saiyan absorbs at least 17,000,000 zenos (unit used to measure waves of full celestial light) through the retina, a reaction occurs in the tail that causes the Saiyans to transform into ruthless Great Apes. If a moon is not present, the Saiyan, if he/she knows how to use it, can also create an artificial moon to allow themselves to absorb the necessary amount of Blutz Waves needed for them to go Great Ape to compensate.
Bulma, while under the influence of Baby during the Baby Saga, creates a machine that can perfectly mimic the nature and effect of natural Blutz Waves. This machine is used to help Baby achieve his Golden Great Ape form, and later is pointed at Vegeta, allowing him to transform into a Super Saiyan 4. The machine can force the transformation even without the tail, but according to Old Kai, this would lead the Saiyan to have absolute control over themselves. A similar device existed on various Saiyan Attack Balls, where they generate a holographic moon that is somehow able to generate Blutz Waves, as evidenced by Goku's Attack Ball and the adverse effects it had on Gohan when he accidentally saw it while training with Piccolo for the fight against the Saiyans.
In the original Japanese version, this phenomenon is referred to as "Bruits Waves" (ブルーツ波浪), which is a portmanteau on the words Brute and Fruits.