A light-year is a distance — not a time. It's how far light travels in one full year.
Light moves so fast it circles the entire Earth seven times in a single second. In a year, travelling at that speed, it covers about 9.5 trillion kilometres.
Our nearest neighbouring star is 4 light-years away. That means the light reaching us from it tonight left before you started reading this — actually, it left 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.