Define a light-year as a distance, place the Sun among the estimated hundreds of billions of stars in the Milky Way, and state the distance to the nearest star system.
Space is so vast that distances between stars are measured in light-years, not kilometers. Light travels about 300,000 km every second, and in one year (about 31,536,000 seconds) it covers about 9.46 trillion km -- that distance is one light-year. The nearest star system, Proxima Centauri, is about 4.24 light-years away, meaning we always see it as it looked 4.24 years in the past.
Our Sun is just one ordinary star among an estimated 100 to 400 billion stars in the Milky Way galaxy, itself just one of an estimated 100 billion or more galaxies in the observable universe. Scientists call these counts estimates because measuring a galaxy's or the universe's full extent precisely is extraordinarily difficult -- but even the low end is a staggering number.
Mastery looks like: They compute a light-year distance from light speed and time, and explain what "estimated" star and galaxy counts mean.
Common stumbles: Thinking a light-year measures time rather than distance; assuming the Sun is unusually large or special compared to other stars.