(1) tre means 3.

(2) corda means "string"

(3) This sentence originally meant "three strings", but now it means to disable the mute pedal. There are three strings for each note on the piano. When using the weak pedal, only one string is struck, which is marked as una corda; when normal playing is resumed, it is expressed as tre corda, which is the mark of the left pedal.

The left pedal is generally labeled with "una corda" (meaning "one string". This is not possible on modern pianos because there are actually "two strings"). When open, it is marked "tre corde" or "tutte le corde" ("three strings").

Extended information:

Application of the left pedal:

Stepping on the left pedal of an upright piano can bring the hammer closer to the strings and reduce the volume. . Therefore, when you need to control the volume but the skill is not up to it, you can use the two strings in the string, and the other string that is not hit will also sound due to ringing, but the timbre will be obviously softer. darken.

When the left pedal is depressed slightly instead of fully depressed, the entire keyboard and action move less to the right than when fully depressed. At this time, although the hammer is not hitting the The three strings in the middle of the note, but because the strings are struck with a less used part of the hammer, they are softer than the frequently used parts where three grooves have appeared, so the sound played will become darker and softer. Make subtle changes in timbre.

Baidu Encyclopedia - Piano Master on Piano Performance