Focusing a telescope when trying to image the Moon and/or planets can be difficult. The Bahtinov mask is a funny looking device that makes the task more objective than trying to decide based upon the display from the camera (whether a "still" camera or video camera). Here is a picture of a commercially available mask that gives an idea of what the thing looks like.
It is quite possible to make one's own from readily available templates on the Internet ( see http://www.focus-mask.com/Free_Templates.html ) and I did so using foam board.
To use, the mask is placed over the front of the telescope and all those slots and bars really play havoc with the images the telescope makes - but in a useful way. The image sequence below of Altair with the mask in front of our 10-in LX-200 telescope shows the way it is used.
In practice, the "X" pattern is more or less constant while the horizontal bar moves up and down relative to the "X" as the focus knob is turned. When the telescope is in focus, the horizontal bar bisects the "X." To use in Lunar/Planetary imaging, the telescope is pointed to a bright star, the mask is used to focus and then the telescope is re-oriented to the object (mask removed, of course).