Focal point of the camera is the 3D position that remains at the same position on the screen display when the camera is rotated.
Focal point is set to the center of the displayed 3D content when you press Reset field of view button. This is the most common way to set the focal point position.
You translate the focal point along the view plane when you pan (translate) the 3D view. To translate the focal point along the view normal there are several options, see details here.
Can you tell a bit about your application? What are you visualizing and what would be the ideal way of setting the focal point?