When humans went to the moon many years ago, they brought a special flag with them to show that they had visited. The flag was made out of special material so that it could withstand the harsh conditions on the moon.
When they got to the moon, they had to assemble the flag because it was in pieces. They had to put the flagpole together first, which was like building a really tall stick. Then they had to attach the special flag to the top of the pole.
After they had assembled the flag, they stuck it into the ground so that it would stay there. This was very important because they wanted the flag to be seen by everyone who looked at the moon from Earth.
So, in short, assembling the lunar flag meant putting together the flagpole and attaching the special moon flag to the top, and then sticking it into the moon's surface so that it would stay there.