A tapered leader runs gradually from thin to thick, so the end you tie to the main line is close to the main line's own diameter. The knot stays small and does not clatter through the rings on the cast. Tapered leaders suit long range carp fishing with heavy leads.
With a standard shock leader, fine main line is joined to thick leader, and the bulky knot can cost distance or wear the guides. With a tapered leader the diameter changes gradually, so the join is tiny.
Tie the thin end to the main line and the thick end to the lead set-up. The heavy section takes the strain of the cast and the abrasion on the bottom, while the fine section peels off the reel freely.
Each tapered leader is sized for one rod and is usually 10 to 15 metres long. Check that the thin end matches your main line diameter. It is the choice when you are casting at range all day and want every cast to flow cleanly.