81:12

And when Hell-fire shall be kindled to fierce ablaze.
Meaning:-
Fierce: powerful, strong, violent, forceful; stormy.
Ablaze: aflame, burning, burning very strongly.

81:15

So verily, I swear by the planets that recede (i.e. disappear during the day and appear during the night).
Meaning:-
Recede: retreat, go back, fade.

81:23

he (Muhammad) indeed saw him (Gabriel) in the clear horizon
Meaning:-
Horizon: the limit or range of a person’s knowledge or understanding.