The black copper cable from the telegraph pole outside your house, which normally goes in to a junction box or an RF/2/3 filter, or directly in to the A+B terminals behind the master socket.
It comes in different sizes for different reasons e.g. an Openreach engineer may use a thicker cable if the cable is rubbing against a tree.
Drop cables are underground in newer build estates.
Drop cables are mostly made of Copper, but can be Aluminium or, in some cases, Steel, where there is a large unsupported length.
Drop cable
