。
Physics is my weak subject.
Literal
Physics [topic-は] my weak subject is.
苦手な (にがてな) means 'weak at' or 'not good at' — a な-adjective. Japanese has this specific word for describing things you're bad at, without the harshness of 'hate' (嫌い) or 'can't do' (できない). A useful learner vocabulary item.