The word attack is way too harsh and forceful, literally and figuratively, to describe what your body does to get a key to go down. But “attack” is so widely used in piano circles that using another word might cause confusion, so it will have to do for now.
A prepared attack means that your fingertip is already touching the key surface when you decide to push the key down.
An unprepared attack means that your fingertip is above the key surface when you decide to push the key down.
Each attack has its advantages and disadvantages:
- Because it provides tactile feedback at your fingertips, a prepared attack breeds enormous confidence that your fingers are on the right notes. But there is a danger that you will start from a static position and use mostly your muscles to push the key down. This can mess up your timing and allow tension to creep in. The way to eliminate this tension is to have some momentum going in your arms and hands before the key goes down.
- Because it starts above the key, an unprepared attack gives your arms & hands & fingers some time & space to smoothly accelerate their weight into the keys. This cultivates a feeling of freedom and physical flow, but there is a danger that your fingertips will land in the wrong spot.
That said, because of the security it provides, a prepared attack is usually preferred, but an unprepared attack may be appropriate in some situations (specifically for large skips at fast tempos when you have not time to prepare). It depends on the musical situation and may arguably be a matter of personal preference and style. Whichever one you choose in a given situation, be aware of the dangers, try to minimize them, and let your coordinated motions fit the flow of the music.