When someone says "a Galaxy is 31 million light-years," they are referring to the distance between us (Earth) and that particular galaxy.
Light-years are a unit of measurement used in astronomy to describe vast distances. One light-year is the distance that light travels in one year, which is approximately 9.46 trillion kilometers (5.88 trillion miles). So, if a galaxy is said to be 31 million light-years away, it means that the light we receive from that galaxy today has taken 31 million years to reach us.
In other words, we are observing the light that left that galaxy 31 million years ago. This concept highlights the immense scale of the universe and the significant time it takes for light to travel across such vast distances.