Buugito (ブギト Bugito) is the result of Super Buu absorbing Vegito. He becomes so powerful, it's said that he can match with Beerus.
After absorbing Vegito, Super Buu gains his Potara-Fusion shirt, as well as his boots. His personality roughly remains the same, aside from becoming much more of a really skilled fighter due to gaining the fighting knowledge of Goku and Vegeta.
Being that Vegito was stronger than Super Buu himself, his power has now more than doubled, making him the currently strongest being in the universe, and thus a very serious threat.
While fighting Super Vegito, Super Buu knew he was losing. Upon taking a blast from Vegito, he left some of his particles in the air, then flew away from Vegito. Vegito, in pursuit, flew after him, with the particles sticking to his shirt. As he prepared to launch another round of attacks on Super Buu, the particles multiplied, quickly covering Vegito and then absorbing him. Super Buu took the particles in, and thus was born as Buugito.