Blutz Waves (ブルーツ波 Burūtsu-ha) 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 (ゼノ Zeno, 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 in various Saiyan attack pods, where they generate a holographic moon that is somehow able to generate Blutz Waves, as evidenced by Goku's attack pod and the adverse effects it had on Gohan when he accidentally saw it while training with Piccolo for the fight against the Saiyans.
When a Saiyan receives extra Blutz Waves, it will increase their power beyond its usual level and also heal their wounds as long as they are receiving Blutz Waves. After Super Saiyan 4 Goku knocks Golden Great Ape Baby out, the Tuffle-parasite infected Bulma uses Blutz Waves in order to fully heal Baby, and also increase his power even further.
It is implied that Black Smoke Shenron is capable of generating blutz waves via his shadows, as he managed to use his shadows to transform Legendary Super Saiyan 3 Broly into a Great Ape and by extension Legendary Super Saiyan 4.