This is the 2194th most frequent Arabic word.
وليست
"وليست" means "and she/it is not."
In this sentence, 'وليست' is used as a negation coordinate conjunction combining two adjective states about the sky.
السماء صافية وليست غائمة.
The sky is clear and not cloudy.
Here, 'وليست' helps contrast between two ideas, rejecting one and affirming the acceptance of the other.
هذه الفكرة وليست الأخرى هي المقبولة.
This idea and not the other is the accepted one.
In this case, 'وليست' is employed to express a desire for one type of solution over another, showing preference by exclusion.