Roughly speaking, a piano's action is light when its keys fall easily under the fingers, and heavy when a noticeable downward thrust is required.
'A professional pianist is likely to care most about the piano's action, because that is what controls its responsiveness and relative lightness-or heaviness-of touch. The design of the key action mechanism determines the 'weighted keys' feeling that is, the feeling of the heaviness of the touch of the keys. Action can refer to that of a piano or other musical keyboards, including the electronic or digital stage piano and synthesizer, on which some models have 'weighted keys', which simulate the touch and feel of an acoustic piano.
The piano action mechanism (also known as the key action mechanism or simply the action) of a piano or other musical keyboard is the mechanical assembly which translates the depression of the keys into rapid motion of a hammer, which creates sound by striking the strings.
