What does full mortise mean?

Both leaves sit recessed into cutouts in the door edge and the jamb, so the hinge is hidden when the door is closed. It is the standard hinge for interior doors.

Will it hold a heavy solid-core door?

This is a standard-weight hinge, so it is meant for interior doors of ordinary weight. Heavy solid-core, oversized or exterior doors call for a heavy-weight hinge.

What do the two ball bearings do?

They sit between the knuckles and carry the door's weight on rolling contact instead of metal on metal. The result is a smoother, quieter swing and less wear than a plain-bearing hinge.

Which size do I need?

Match the size to the existing cutouts and the door's weight; 4 x 4 is common on lighter interior doors, and 4.5 x 4.5 or 5 x 5 on larger, heavier ones. Measure the old hinge leaf before ordering.

Do the screw holes line up with my old hinge?

The holes follow Stanley's template pattern, which matches most standard door preps. If your door was drilled for another brand, check the hole spacing against the old hinge first.