From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Bruits Waves (ブルーツ波, Burūtsuha; FUNimation "Blutz Waves"), Green-spectrum Radiation, are an energy source emitted by large celestial bodies, such as a full moon or Planet Earth (in Goku's case in Dragon Ball GT). When a Saiyan absorbs at least 70,000,000, or 17,000,000, Zeno of Bruits Waves through the retina, a reaction occurs in a Saiyan's body which, catalyzed by his tail, causes the Saiyan to transform into a ruthless Great Ape.
The name is a pun on the English words brute and fruits.
Dragon Ball GT
Bulma, while under the influence of Baby during the Baby Saga, created a machine that could amplify the effect of Bruits Waves to such an extent that a tail was not needed to achieve transformation. They were called Chō-burūtsuha (超ブルーツ波, Chō-burūtsuha; Literally meaning "Ultra Bruits Wave"). This machine was used to help Baby achieve his Great Ape form, and later was pointed at Vegeta and allowed him to transform into his Super Saiyan 4 form.
It appears that an artificially high concentration of Bruits Waves could also be used to re-energize a Great Ape. Using the same Bruits Wave Amplifier, Bulma was able to bring Great Ape Baby back to full power after he was knocked out by Goku.