Gagaku is a centuries-old Japanese tradition of court music and dance. It includes instrumental music, songs, and formal dances performed in elaborate costumes.
Its distinctive sound is created by instruments such as the shō, hichiriki, and ryūteki. Many Japanese people would associate that sound with the imperial court, shrines, temples, and formal ceremonies, even if they could not name the instruments. It can sound calm and beautiful, but also mysterious and far removed from everyday music.
Not all of gagaku originated in Japan. Music and dances brought from China, the Korean Peninsula, and other parts of Asia were adapted within the Japanese court over many centuries. Through that process, they developed into the tradition now known as gagaku.
Although most Japanese people know what gagaku is in a general sense, few encounter it regularly. I have never attended a live performance myself, and I cannot remember personally knowing anyone who was especially familiar with it. My image of gagaku comes mainly from solemn performances at court ceremonies, shrines, and temples. Naturally, people who study or perform traditional music experience it very differently and know its forms and instruments in much greater depth.
During a ceremony, gagaku may not feel like a concert presented for its own sake. Instead, it becomes part of the ceremony itself, giving the occasion a formal and solemn atmosphere.
For many Japanese people, then, gagaku is a well-known part of Japanese tradition, but not a familiar part of everyday life.