The law of impermanence is a fundamental premise of Buddhism. Our reality we experience is in a constant state of flux. In the photo you can see rain and dark clouds coming but right behind the sun is shining. These are tangible changes that are easy to see especially when you are running down the mount getting soaked and by the time you get to the bottom the sun is out :)
But when you sit at a rivers edge it becomes less clear.
The river can appear to be the same to the naked eye, but in reality it never is. Life is much the same. One moment, succeeded by another, in a successive flow that gives the appearance of a continuous flow.
On an individual level we are like the river. We have the appearance of continuity. The same Paul that woke up this morning will be the same Paulthat will go to sleep tonight.
On a biological level we know that is only a partial truth. Cells are constantly passing away
and being replaced with new. The flowers on the table in front of me are dying albeit so slowy that I cannot experience it in the moment, but tomorrow they will be different.
Impermanence is an undeniable truth but why did the Buddha make such a big deal about it?
Long story short suffering comes from being caught in attraction and aversion to that which changes. This results in a constant wrestle with existence.
The practice of mindful attention to the impermanence of everything takes place in a state of stillness. This practice of non-judgemental observation of the changes in body, mind and external reality, can loosen the knots that bind us to our ideas of a fixed sense of self.
Practice dropping one moment completely and welcoming the next and then dropping that just as quickly, continuously 24/7. Let me know how you get on :)