Do you know what it means when we say "Distance"? It’s how far apart two things are from each other. Right? Same goes for the moon and the Earth. They are not really close - about 238,855 miles away from each other.
So, when astronomers say "Lunar Distance," they mean the distance between the moon and Earth.
Astronomers like to use Lunar Distance to measure how close an object in space is to us. For example, did you know that the International Space Station orbits Earth and is only 250 miles above the planet's surface? That's really close compared to the 238,855 miles between Earth and the Moon.
People also use Lunar Distance when they talk about things like comets or asteroids that might come close to Earth. By measure how many times the distance between the moon and Earth an object is, they can tell just how close it's going to get to us.
So, that's what Lunar Distance means, and why astronomers use it to talk about how far away things are from Earth.