x variable is going to be used within for loop, as an index of the array myArr. That array's size has been defined at the time of declaration. 5. So we are going to loop for x until it reaches 4 (x<5). because array's index are 0 based. so last element (5th) will be at the index 4.
In every loop, myArr[x] is 42. Since x will be from 0 to 4, myArr's every element will be 42.