 he means that they ride the desert at the end of the night when the galaxy shifts from its place and he called it the mother of stars because of the gathering of stars in it and their abundance. it is said that the stars came close together in the galaxy so some of them obscured others so it became like clouds.
يريد أنهم يركبون الفلا آخر الليل عند تحول المجرة عن موضعها وسماها ام النجوم، لاجتماع النجوم فيها وكثرتها. ويقال إن النجوم تقاربت فى المجرة، فطمس بعضها بعضا، فصارت كأنها سحاب.