This is the 353rd most frequent Arabic word.
ستكون
"ستكون" means "will be."
Here, "ستكون" is used as a future tense auxiliary verb indicating that the sky will have the state of being clear tomorrow.
ستكون السماء صافية غدًا.
The sky will be clear tomorrow.
In this case, "ستكونين" is conjugated for the second person singular feminine, showing future tense.
ستكونين سعيدةً برؤية أصدقائك.
You (female) will be happy seeing your friends.
Here, "ستكون" indicates a future condition describing the nature of the upcoming trip.