I wanted to thank you for the help that you have me already brought in my precedent post, today I still need your advices to end the writing of my code:
I don t know if it s possible but I would like that we can see a track ( a line) between my circles when I move my mouse..
As if my circles could draw a line as one goes along in an reinterpretated style of Jackson Pollock and my circles would remain evanescent like it already are here..
I hope you understand me ^^'

I also noticed that my Void mousepressed did not work well because I wanted by clicking on A, it would appear a black circle which wouldn t remain on the background or it doesn't stay on the background.
Here is my code...

int click = 0;
int lenght = 200;

void setup() {
  size(500, 500);
  background(255);
  smooth();
}
 
//declare arrays for mouse position in x and y, length
float x[] = new float[lenght];
float y[] = new float[lenght];
 
 
void draw() {
  background(255);
 
 
  //reads entire array
  for (int i=1; i<100; i++) {
    //the first value is deleted, all values are moved one position down
    x[i-1] = x[i];
    y[i-1] = y[i];
  }
  //mouse x and y are the new last item of the arrays
  x[100-1] = mouseX;
  y[100-1] = mouseY;
 
  //read again the array, with new values
  for (int i=0;i< x.length; i++) {
    //dessin du cercle
    noStroke();
    fill(0, 0, 0, 100);
    ellipse(x[i], y[i], i-30, i);
 
    strokeWeight(4);
    stroke(random(255, 255), 255);
 
   
  }
}
void keyPressed() {
  if(key=='a'){
    fill(255);
  x [click]=mouseX;
  y [click]=mouseY;
  click++;
  }
}

Thank you for taking time to answer me and to explain my errors:)