To reef a sail, roll it up on itself and bundle the material along a boom or mast. This reduces its effective area, which, apparently, proves useful in high winds. I learned of this word in the Friday NY Times Crossword Puzzle of December 1, 2023.
Reefing reduces the area of a sail, usually by folding or rolling one edge of the canvas in on itself and attaching the unused portion to a spar or a stay, as the primary measure to preserve a sailing vessel’s stability in strong winds. Restoring full sail area is termed shaking out a reef.
Wikipedia contributors, “Reefing,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Reefing&oldid=1097456005 (accessed December 1, 2023).