Pigtailing is combining two or more wires with an additional wire which will be the one to attach to a terminal. Screw terminals that clamp down on wires you insert in nearby holes often let you connect more than one wire there, so that there is no need to pigtail. But more often the screws only take wires curled clockwise directly under themselves; then only one wire should be put under each screw, and so it may be necessary to pigtail so that all the (say, white) wires of the circuit connecting there have contact with each other. On some devices there are (additional) holes for terminating wires by simply pushing wires in till they catch. If there are enough such holes, pigtailing will not be needed. Such holes on receptacles will not accept large (12 gauge) wires, in which case the wires will need to be pigtailed if there are not enough screws to put them all under. An example of pigtailing diagrammed.