The shrouds are actually a number of lines.
The top is a small platform at the top of the lower mast on a ship. The purpose is to provide anchor for the topmast shrouds. If the topmast shrouds were to be attached to the hull of the ship, the support would not be wide enough to properly support the mast. The top spreads the bottom of the topmast shrouds so it better supports the topmast.
Construction is something like this.
The trestle trees are mounted on the mast fore-and-aft, athwart between them against the mast are the chocks to keep the trestle trees parallel.
The crosstrees are mounted on top of the trestletrees athwartships, parallel to the chocks. Crosstrees are notched into the trestletrees.
Bolsters lie on top of the trestletrees between the crosstrees, the top surface is on the same plane as the crosstrees.
Planks are then laid athwartships on top of all that to provide the surface of the top. Ribs are laid radially beneath the top to provide strength and stiffness to the top.
The lubber's hole is cut into the top for no one to use except landsmen.
A rail is attached to the edge of the top.
The top is then supported by the futtock shrouds from below and the shrouds supporting the lower mast is anchored just below the top.
Tops are named for the topmast it supports, foretop, maintop, mizzentop.
Tops are also used by crewmembers as a lookout point. The top usually has a hole in it to gain access to the top, known as the 'lubber's hole', but most crewmembers refuse to use it and gain access by climbing the futtock shrouds.
If the top is enlarged and a swivel gun mounted, it is called a fighting top.
Glossary: SearchWikipedia
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