one's field of expertise; one's strong area
Refers to a subject, skill, or domain in which a person excels or has special knowledge.
His field of expertise is mathematics.
I look for a job that makes use of my strong area.
専門分野 is a more formal term for a specialized field of study or profession, while 得意分野 emphasizes personal strength or proficiency.
Compound of 得意 (とくい, 'strong point') and 分野 (ぶんや, 'field').