A fireball, which is another term for a very bright meteor, has an apparent magnitude that is brighter than -4, according to the American Meteor Society.
Minus 4 is the same magnitude of the planet Venus as seen from the surface of the Earth.
The phenomenon of fireballs are quite common. Several thousand meteors of fireball magnitude occur in the Earth’s atmosphere each day.
Not many are seen, however, because the vast majority of fireballs take place in the skies above the oceans or uninhabited regions.
A large number of fireballs also take place during the day, when sunlight camouflages them.
Because people generally stay indoors during night time, there is a smaller chance that they will be seen.
Another factor which makes seeing a fireball rare is the magnitude.
Experienced observers can expect to see only about 1 fireball of magnitude -6 or better for every 200 hours of meteor observing, while a fireball of magnitude -4 can be expected about once every 20 hours or so.
Source: American Meteor Society