A fiber optic pigtail is a short fiber cable with a factory-terminated connector at one end and a bare fiber at the other. The bare end is fusion-spliced to a field cable fiber to form a permanent termination, while the connectorised end plugs into the panel adapter; from there, connections are made with patch cords.
Pigtails exist because of termination quality. When a connector is assembled in the field, polishing, alignment and cleanliness cannot be controlled, so insertion loss is both higher and inconsistent. A pigtail connector is assembled and measured in the factory, giving low, repeatable loss. The only field operation left is the fusion splice, which is well controlled.
Selection covers fiber grade, connector type and length. The pigtail grade must match the field cable: OM3 to OM3, OS2 to singlemode. Connector type matches the panel adapter (LC, SC or ST). Length is most often 1 metre, long enough to leave slack storage inside the tray yet short enough to avoid clutter.
After splicing, the joint is protected with a splice protector and seated in its slot inside the tray. Coiling fibers within their bend radius in the tray matters so later work does not damage them. Colour coding helps here too: aqua marks OM3/OM4, orange OM1/OM2 and yellow singlemode links.