Stays are the heavy ropes, wires, or rods on sailing

Sailing

Sailing is the propulsion of a vehicle and the control of its movement with large foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to move the boat relative to its surrounding medium and...

 vessels that run from the masts

Mast (sailing)

The mast of a sailing vessel is a tall, vertical, or near vertical, spar, or arrangement of spars, which supports the sails. Large ships have several masts, with the size and configuration depending on the style of ship...

 to the hull

Hull (watercraft)

A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

, usually fore-and-aft along the centerline of the vessel. The stay that runs aft is called 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...

 and the stay that runs forward is called forestay

Forestay

On a sailing vessel, a forestay, sometimes just called a stay, is a piece of standing rigging which keeps a mast from falling backwards. It is attached either at the very top of the mast, or in fractional rigs between about 1/8 and 1/4 from the top of the mast. The other end of the forestay is...

 or just stay.

Lines running from the mast to the side of the boat are usually called shrouds

Shroud (sailing)

On a sailboat, the shrouds are pieces of standing rigging which hold the mast up from side to side. There is frequently more than one shroud on each side of the boat....

 instead.

To "miss stays" is an unsuccessful attempt to tack.

Types of stays

"A stay is part of the standing rigging (as opposed to the running rigging) and is used to support the weight of a mast. It is a large strong rope extending from the upper end of each mast and running down towards the deck of the vessel in a midships fore and aft direction. The shrouds serve a similar function but extend on each side of the mast and provide support in the athwartships direction. The object of both is to prevent the masts from falling down but the stays also prevent springing, when the ship is pitching deep. Thus stays are fore and aft. Those led aft towards the vessel's stern are back-stays while those that lead forward towards the bow are fore-stays. For example, one fore-stay reaches from the foremast-head towards the bowsprit end. The main-stay extends to the ship's stem. The mizzen-stay stretches to a collar on the main-mast, immediately above the quarter-deck. The fore-topmast stay goes to the end of the bowsprit, a little beyond the fore-stay, on which the fore-topmast staysail runs on hanks. The main-topmast stay attaches to the hounds of the foremast, or comes on deck. The mizzen-topmast stay goes to the hounds of the main-mast. The top-gallant, royal, or any other masts, have each a stay, named after their respective masts. Spring-stay is a kind of substitute nearly parallel to the principal stay, and intended to help the principal stay to support its mast. On a steamer, an iron bar between the two knees secures the paddle-beams. (See funnel stays).

"To stay. To tack, to bring the ship's head up to the wind for going about; hence to miss stays, is to fail in the attempt to go about.
In stays, or hove in stays, is the situation of a vessel when she is staying, or in the act of going about. A vessel in bad trim, or lubberly handled, is sure to be slack in stays, and refuses stays, when she has to wear."