Magnitude is a number that tells you how bright an object is, and it can be relative or absolute. Vsauce explores the Universe of absolute magnitude and finds out that the Sun has a magnitude of 4.83, yet a star called R136a1, 256 times more massive than the Sun, has a magnitude of -12.6 and 8.7 million times brighter than our Sun. Lower absolute magnitudes mean more brightness.
Vsauce posted a link to a magnitude calculator that will be handy to me in the future. So, the more you go into negative magnitudes, the brighter that thing is. Is R136a1 the brightest thing in the Universe? Not even close.
The brightest thing in the Universe is the quasars, the first discovered one being 3C 273 with a magnitude of -26.7, which means that it is 4 trillion times more brighter than the Sun. If you would put 3C 273 at a distance of 33 light years away it would shine as bright as the Sun.
The quasars are black holes that eat up matter and while it is eating up that matter a disk forms around them (accretion disk). These disks spin around the black hole at great speeds while the matter from within if engulfed in the black hole. This rotation generates a lot of heat and light and thus we notice it by how bright it is.
Quasars are thousands of times more brighter than the galaxies with billions of stars they’re in. Oh, and they generate also radiation jets that could affect planets thousands of light years away. Wow!