You need to start reading the PDF specification, you will not get far by asking questions here about each pdf operator.
1. The q operator saves the graphics state, the Q operator restores it. In your context 'q q' is redundant.
2. The matrix sets the image (/Im0 that follows) position and size. Elements 1 and 4 of the matrix are image size, elements 5 and 6 and image position (bottom left corner). This is a simplistic explanation, you should read the PDF specification to see how the trasnformation matrix works.
3. The Do operator paints the /Im0 XObject (your image).
4. % starts a comment line and % DSBlank is just a comment.
PS: The application you use for exploring the PDF structure is developed by the company I work for :)