Dynamics (how loudly or softly something is played) is a huge component of the expressive power of music…
Piano & Forte
The two most commonly-used indications are “piano” and “forte” as well as their variations…

Piano trivia: The indications “piano” and “forte” are the namesakes of the piano, which was invented in the early 1700s in Florence by an Italian named Bartolomeo Cristofori. The name piano is a shortened version of its original Italian name: piano-forte (literally “soft-loud”). The ability to play soft and loud required a mechanical action of revolutionary complexity and was a major advance in keyboard instrument design at the time.
Crescendo & Diminuendo
Gradual increases and decreases in volume are indicated with diverging or converging lines, respectively…
