Mu'awwal is wording for which reliable evidence identifies one intended meaning from the possible meanings.
The source uses the Quranic expression ثَلَاثَةَ قُرُوءٍ - “a period of three quru'” - as an illustration:
• قُرُوءٍ can mean menstrual periods or periods of purity. This is the ambiguity.
• A qualified Mujtahid is a scholar trained to examine the relevant proofs.
• In the source's example, that evidence-based examination selects menstruation as the intended meaning.
• After this selection, the wording in that analysis is called Mu'awwal.
The learner is understanding the reasoning chain, not performing Ijtihad or deriving the family-law ruling.