This is a short piece of cable from the solar panel junction box to a grey 50A 2 pole connector, which allows for easy connect / disconnect of the solar panel to the regulator, a second panel, an extension cord, etc.
Generally, a very short length is sufficient, but longer lengths can be made.
Due to the wide variety of solar panels and junction boxes, the price charged for the pigtail is a general averaged price regardless of what is used for a specifc solar panel. The length of cable may vary slightly, sometimes lugs are used, and sometimes glands are necessary. The cable used is 4mm2 ripcord.