I never learned to read music properly; I just play everything on the guitar by ear.

play 在这里是动词，意思是"演奏（乐器）"。 by 是介词，表示"通过某种方式或手段"。 ear 指"耳朵"，这里指听觉能力。 play by ear 是一个固定短语，字面意思是"用耳朵演奏"，实际指"不看乐谱，凭听觉记忆演奏音乐"。