Light is the main signal we receive from space. Almost everything we know about space has been found by measuring light. One of the main ways to measure light is to look at the brightness. Measuring the brightness of something in space isn't as easy as you'd think though. It can be done in a few different ways.
Often magnitudes are used to describe the brightness of an object. They come in 2 forms: absolute and apparent. This is because how bright an object in space looks depends on how far away from Earth it is. The apparent magnitude is how bright the object appears from Earth. The absolute magnitude is how bright the object would be if it were placed at a set distance from the Earth of 10 parsecs.
Brightness in astronomy can also be measured using luminosity. This is a more physical measurement. It measures the energy output of the object. This is done by looking at how much light is given out by the object over time. The units of luminosity are more standard, given in Watts - much like a light bulb! A typical LED light bulb in a house might be 20 Watts - the Sun is 20 million billion billion (2 x 1025) times brighter!