He/She is someone who specializes in writing articles about interior design.
"He/she is someone"＝「彼（彼女）は～をする人です」
"who specializes in"＝「～を専門とする人」
"writing articles about interior design"＝「インテリアデザインに関して記事を書くこと」
Sentence in context:
"I write articles for magazines, but not just on any topic. I specialize in writing about interior design."
アメリカ合衆国 
