We could start by asking the question, what are we actually hearing? This question is more relevant than you think. Boulez draws inspiration from many sources. He admits to being influenced by Ravel, Stravinsky and Bartok for this set of piano pieces.
Listen to this music and think about what it reminds you about. For some reason, when ever I hear this, my mind subconsciously adds a layer of swinging jazz rides and brushed snare to the music. What does it make you feel? Even if your answer is "It doesn't make me feel anything", it's a legitimate response. The more interesting question now: why does it make you feel the way it does?
In the last video, I played the Notation 2, which is in Stravinsky style and almost made my fingers bleed. Check it out if you've missed it. The next video covers the next notation which, for its minute length, covers ideas such as permanence vs impermanence.