Question

 public class Sprite 
 {
   private static final int BMP_COLUMNS = 3;
   private static final int BMP_ROWS = 4;
   private int x= 0;
   private int y= 50;
   private int xSpeed= 5;
   private GameView gameView;
   private Bitmap bmp;
   private int width;
   private int height;
   private int currentFrame = 0;

 public Sprite(GameView gameView,Bitmap bmp)
  {
    this.gameView=gameView;
    this.bmp=bmp;
    this.width = bmp.getWidth()/BMP_COLUMNS;
    this.height = bmp.getHeight()/BMP_ROWS;
   }
 private void update()
 {
     if(x > gameView.getWidth() - width - xSpeed)
     {
         xSpeed = -5;
     }
     if(x + xSpeed<0)
     {
         xSpeed=5;
     }
     x = x + xSpeed;
     currentFrame = ++currentFrame % BMP_COLUMNS; <--how it works..?
    }

 public void onDraw(Canvas canvas)
      {
     update();
     int srcX = currentFrame*width; <---how it works..?
     int srcY = 2*height;   <-----how it works..?
     Rect src = new Rect(srcX,srcY,srcX+width,srcY+height);  <--how it works..?
     Rect dst = new Rect(x,y,x+width,y+height);  <----how it works..?
     canvas.drawBitmap(bmp,src,dst,null);        
     }
 }

Answer 1

The source bitmap is a composite of 3*4 sprite images.
This sprite uses 3 different images in the bitmap and the code will cycle through them (image 0, 1, and 2).

currentFrame = ++currentFrame % BMP_COLUMNS; <--how it works..?

currentFrame will cycle through the values 1..BMP_COLUMNS-1, i.e. 0, 1, 2, 0, 1, 2, …

int srcX = currentFrame*width; <---how it works..?
int srcY = 2*height;   <-----how it works..?

Finds the position of the image to use. For this particular sprite there are 3 images (which probably are almost identical, maybe the difference is a blinking light or something like that).

Rect src = new Rect(srcX,srcY,srcX+width,srcY+height);  <--how it works..?

Calculates the position in the source bitmap.

Rect dst = new Rect(x,y,x+width,y+height);  <----how it works..?

Calculates the position on the display.