Stars are the source of light in space. Stars use nuclear fusion to shine. Brightness depends on a star's size and temperature. Hotter and bigger stars shine more brightly. Other objects in space are lit by reflecting light from stars. For example, the moon is illuminated by the sun shining on it.
Apparent brightness is a star's brightness as seen from Earth. However, brightness cannot be compared, since it varies based on distances. Absolute brightness, luminosity, is the brightness of a star at a standard distance from Earth.