Technique
Tuning
Adjusting pitch so notes match an agreed reference.
In plain English
Tuning can mean adjusting an instrument before playing or controlling pitch while a note is sounding. Singers and fretless players continually tune by ear.
Try or listen for this
Adjust an A string until it matches a tuner or another instrument playing the same A.
Why it matters
Good tuning allows notes and instruments to blend without unwanted beating or clash.