This option makes sense only in polygon mode. Enabling the option means that an edge or polygon will be selected if any part of it is inside the selection marquee. If the option is disabled, the polygon or edge will only be selected if all of its corner points are inside the marquee.
When this option is enabled you can select only polygons that are not behind other polygons. For example, if you are selecting polygons at the front of a sphere, polygons at the back of the sphere will not be selected, even if you are working in wireframe mode and can see the polygons at the back.
Another way to prevent polygons at the back from being selected is to switch on
In addition, you can also hide polygons to prevent them from being selected using the Hide Selected and Hide Unselected commands from the Selection menu.
Lastly, set an object’s top visibility dot to red to hide that object and to prevent its elements from being selected. (You will find the visibility dots in the Object Manager, next to the object’s icon.)