The topping lift is a line

Sheet (sailing)

In sailing, a sheet is a line used to control the movable corner of a sail.- Fore-and-aft rigs:Fore-and-aft rigs comprise the vast majority of sailing vessels in use today, including effectively all dinghies and yachts. The sheet on a fore-and-aft sail controls the angle of the sail to the wind,...

 which is part of the rigging

Running rigging

Running rigging is the term for the rigging of a sailing vessel that is used for raising, lowering and controlling the sails - as opposed to the standing rigging, which supports the mast and other spars....

 on a sailboat

Sailboat

A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in the size are not strictly defined and what constitutes a sailing ship, sailboat, or a...

; it applies upward force on a spar

Spar

In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a...

 or boom

Boom (sailing)

In sailing, a boom is a spar , along the foot of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail. The primary action of the boom is to keep the foot of the sail flatter when the sail angle is away from the centerline of the boat. The boom also serves...

. The most common topping lift on a modern sailboat is attached to the boom.

Topping lifts are primarily used to hold a boom up when the sail is lowered. http://www.sailingusa.info/weather_and_reefing.htm This line would run from near the free end of the boom(s) forward to the top of the mast. The line may be run over a block at the top of the mast and down to the deck to allow it to be adjusted. For small booms, the topping lift may be run from end of the boom to the backstay

Backstay

On a sailing vessel, a backstay is the piece of standing rigging that runs from the mast to the transom of the boat, counteracting the forestay and jib...

 or next mast aft. When the sail is raised again, the topping lift is loosened or removed. On sailboats used for racing, boom lift function is frequently incorporated into the boom vang

Boom vang

A boom vang or kicking strap is a line or piston system on a sailboat used to exert downward force on the boom and thus control the shape of the sail. An older term is "martingale"....

 system, to reduce the number of lines aloft. Allowing the vang to take this function also simplifies operation of the boat. http://www.sail-world.com/CruisingAus/index.cfm?SEID=2&Nid=21818&SRCID=0&ntid=0&tickeruid=0&tickerCID=0

A topping lift may also refer to a line on the front of the mast used to rig the spinnaker pole. It is used to trim the pole to the proper height when hoisting the Spinnaker

Spinnaker

A spinnaker is a special type of sail that is designed specifically for sailing off the wind from a reaching course to a downwind, i.e. with the wind 90°–180° off the bow. The spinnaker fills with wind and balloons out in front of the boat when it is deployed, called flying. It is constructed of...

.