A Light year is the distance that light travels in one year. Light travels 186,000 miles per second or just under 300,000 kilometres per second and nothing can travel faster. That means by the standards of the Earth and the Solar System light can travel very far in a year.
The nearest star to the Solar System is just over 4 light years away. Light takes just over 4 years to travel from Earth to Proxima Centauri. The whole Milky Way Galaxy is about 100,000 light years across. The nearest large galaxy is the Andromeda galaxy and that is about 2 Million light years away. Most other galaxies are millions or Billions of light years away. 
Light year can also mean a very long period of time or a metaphorical distance especially in development or progress. 
Conservative ideas are light years away from anything up to date, decent or even reasonable.
- Light years Astronomy video.