What is that indicating how loud or soft to play?

The indication of how loud or soft to play in music is referred to as dynamics. Dynamics are typically represented by specific musical symbols or terms, such as "p" for piano (soft), "f" for forte (loud), and various markings like "mf" (mezzo-forte) or "pp" (pianissimo) to denote gradations in volume. These instructions guide performers on the appropriate intensity to convey the emotional content of the music.