The piano's touch or 'touch weight' refers to the pressure required to press a piano key.
Too light of a touch, or touch that is uneven from note to note, makes a piano action difficult to control; too heavy of a touch makes a piano feel like 'playing a truck', tiring to play, and can cause physical problems for the player over time.
The piano technician measures and analyses the exact touch weight in every key and makes fine adjustments by changing the weight and balance of each key.
The result will be a piano that will play silky smooth and evenly. Some pianists describe the results as 'a touch to die for' or 'it almost plays itself'.