Careening a sailing vessel is the practice of beaching it at high tide. This is usually done in order to expose one side or another of the ship's hull for maintenance and repairs below the water line when the tide goes out. This practice is also known as to "hove down".
The process could be assisted by securing a top halyard to a fixed object such as a tree or rock to pull the mast over as far as possible. Maintenance might include repairing damage caused by dry rot or cannon shot, tarring the exterior to reduce leakage, or removing biofouling organisms such as barnacles to increase the ship's speed. One exotic method was the ancient practice of beaching a ship on a shingle beach with the goal of using wave action and the shingle to scour the hull.
A beach favoured for careening was called a careenage. Today, only small vessels are careened, while large vessels are placed in dry dock.
A related practice was called a Parliamentary heel in which the vessel was heeled over in deep water by shifting weight to one side - as in ballast and guns - causing the vessel to list. In this way the upper sides could be cleaned or repaired with minimal delay. Famously, the HMS Royal George was lost while undergoing a Parliamentary heel in 1782.
Careening in popular culture:
Pirates would often careen their ships because they had no access to drydocks. A secluded bay would suffice for necessary repairs or hull cleaning, and such little "safe havens" could be found throughout the islands in the Caribbean and nearly around the world. One group of islands, Tres Marias, became popular when Francis Drake had sailed there in 1579 and quickly became a place for piracy.
Robert Louis Stevenson's Treasure Island contains a reference to the practice: the Hispaniola is purposely beached on the island. Although the purpose of this is to avoid the uncertainties of anchoring her with nobody aboard, that a piratical crew member would be quick with the suggestion--and the means of freeing the ship later--shows his familiarity with the practice.