Android SurfaceView is an object that is associated with a window (but behind a window), using which you can directly manipulate the canvas and draw whatever you like.
What is interesting about the implementation of SurfaceView is that although it lies BEHIND a window, as long as it has any content to show, Android framework will let the corresponding pixels on that window to be transparent, thus making the surface view visible.
It is most likely to be used for building a game or browser, where you want a graphical renderer to calculate pixels for you while you can also use java code to control the normal APP logic.
If you are new to normal Android programming, chances are you do not need to know too much about it.
For further information, see this and the official documentation.