وَحِفْظًا مِّن كُلِّ شَيْطَـٰنٍ مَّارِدٍ
Wa hifzam min kulli Shaitaanim maarid
and made them a safeguard against every rebellious devil:
and as a guard against every rebellious devil,
Two jobs are given to the same objects in consecutive lines: the stars adorn the lowest heaven, and they also fortify it. Mārid describes something stripped of restraint, a word used of a bare tree or a smooth wall with nothing left on it to grip, and the blocked eavesdropping in the lines after this sets up the challenge the surah puts to its deniers a few verses on.