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A light year is the distance light travels in a year. It's about 5.87 trillion miles.
The light we see from Andromeda left Andromeda 2.3 million years ago. So when we look at Andromeda, we're looking back in time 2.3 million years.
Andromeda is visible to the naked eye as a dim, fuzzy cotton ball between Pegasus and Cassiopeia. It's the most distant thing you can see with your naked eye (at least in the Northern Hemisphere).