This is the 637th most frequent Arabic word.
سبب
Cause; reason.
Here, 'سبب' (reason) is used as a noun indicating causality.
كان السبب في تأخره زحمة المرور.
The reason for his delay was the traffic.
In this sentence, 'سبب' is a noun used to inquire about the cause of a specific state (happiness).
ما هو سبب سعادتك اليوم؟
What is the reason for your happiness today?
Here, 'سبب' appears as a noun denoting the origin or cause of an issue.