You can read about Processing's commands @ Reference
println()   -> http://processing.org/reference/println_.html
random() -> http://processing.org/reference/random_.html

Those 2 println() above are to inform you about pokerface.jpg dimensions.
You see, variable lol stores the reference of that loaded picture file.
Variable lol itself is an instance of class PImage object.

You can access both width & height of an PImage variable type
through its 2 field variables -> width & height:
lol.width & lol.height.

random() picks a number between a range by chance.
So float r = random(255); chooses some float number between 0 & 255 and stores it into variable r.
Although 255 itself is never reached!

Later, this random value is used in fill() to change the filling paint color of the following rect().

Any further doubts, just ask!  

Please use a different title in the future