A light-year is a distance, not a time — it's how far light travels in one year.
Light is the fastest thing in the universe, so fast it circles the entire Earth 7.5 times in a single second. The Sun is 8 light-minutes away — that means the sunlight you feel right now actually left the Sun 8 minutes ago.
In a year, travelling at that speed, light covers about 9.5 trillion kilometres. Even so, our nearest neighbouring star is over 4 light-years away. The light reaching us from it tonight left long before you started reading this — in fact, over four years ago.
The planets in this guide range from a handful of light-years away to thousands. The further the planet, the further back in time the light you're seeing actually came from.