void setup() 
{
  Serial.begin(115200);
  for(int i=3;i<8;i++) //since startin value is 3 and since 3 less than 8 is true, go inside the loop
  {                           // 2nd iteration i is 6 now
    i++;// i become 4         2nd iteration i is now 7
    Serial.println(i); // print 4       2nd iteration print 7
    i++; // i become 5        2nd iteration i is now 8

  } // since 5 less than 8, increase i by 1(i++), now i is 6. go back to beginings(repeats)        2nd iteration since 8 is not less than 8 and exit the loop
}

void loop() 
{
  //Serial.println("loop"); 
}