The "Snap Point" is the offset in a sound that is used with the Snap Settings. It is drawn as a red line. You can move it by positioning the mouse on the bottom of the red line.
For example, if you had a sound with a long introduction and you wanted the sound to snap to that first beat, you could move the snap point to the first beat. Alternatively, you can right click on the loop editor wave form and choose "Set As Snap Point" to set the "Snap Point".
By default, the "Snap Point" is at time 0.