One's area of expertise or strong suit — a field in which a person is particularly good or skilled.
彼女の得意分野はデータ分析だ。
Her field of expertise is data analysis.
就職活動では、自分の得意分野を明確に説明できることが重要だ。
In job hunting, it's important to be able to clearly explain your area of strength.
英語は彼の得意分野で、プレゼンも自信を持って行える。
English is his strong area, and he can give presentations with confidence.