Sometimes, some customers having in their minds purchasing a grand piano would ask this question: 'Is it broken?' when they are testing the piano in a piano shop.
Well, they ask this question when their foot is placed on one of the pedals and they see a horrifying phenomenon happening before their eyes: THE KEYS ARE MOVING…
Yes, dear customers, it is quite normal.
In fact, if you pay attention when you test all the pedals on the grand piano, you will notice that the keys are only moving when the left pedal (aka soft pedal) is pressed down. And this entire key-shifting-to-the-right is due to the actions shifting sideways so that treble hammers only hit 2 strings instead of 3 strings.
WAIT…
Actions?
Treble hammers?
2 or 3 strings?
Have you ever looked at the inside of a piano and noticed a lot of strings were set in a fixed place? And below (or before, depends on the piano types) the strings are a set of wooden material that looks like a hammer with oval shaped head? 
And you have probably also noticed, besides strings and hammers, another set of wooden materials rest upon the strings. These are the dampers, each of which is a small wood piece covered with soft felt that normally presses against the strings to prevent them from vibrating.
When you hit a piano key, a series of mechanism behind-the-scenes causes the following two things to happen simultaneously:
1. Hammers to hit the strings and rebound, 
2. Dampers lifted away from the strings,
The result of the above 2 events: string vibrates freely, generating a sound.
A 'NOTE'  in the piano is usually generated by the hammers hitting a set of 3,2, or 1 string. 
Now that you have primitive understanding on how a tone is produced in piano, we can define the term ACTION.
An action is defined as the intricate mechanism in a piano that transmits motion from the keys to the hammers and dampers. As a result, the term action is commonly used to indicate the keys and all intricate parts that convey motions to hammers and dampers.
Go back to the top of this article again and see if you understand the explanation.
Because the hammers are slightly shifted to the right as the left pedal is pressed down, the note that is usually generated by the hammer hitting 3 strings now is generated by the hammer hits only 2 strings, results in softening, mellowing tone.
Hence, the soft pedal in the grand piano is also called the Una Corda pedal.
Next time when you test a grand piano in a piano shop, don't be surprised to see the keys moving!