"Learning the ropes" has become a modern idiom, but it's rooted in the era of sailing ships when apprentices needed to be able to identify each one of the many ropes on board — for clarity, fast action, and safety. Today, ropes are often called "lines" on boats, and there are a few worth memorizing so you're ready to give or follow clear commands. When powerboats or sailboats come alongside a dock, you'll tie up using a "bow line," a "stern line," and a "midship line." All are attached to the boat using "cleats" — metal fittings shaped like two horns and fixed to the boat; lines are secured to them.
On sailboats, a "halyard" is a line used to hoist a sail up the mast; there's a mainsail halyard, for example, and a jib halyard. A "sheet" is a sail-control line that's normally controlled by wrapping it around a "winch"; a sheet holds the bottom part of a sail tight so it can use the wind to propel the boat. If you're asked to "sheet in the jib," it means the skipper would like you to turn the winch holding the jib sheet with the winch handle, and trim (pull in) the sail a bit more. If he or she asks you to "let the sail out," with the winch you'd ease the jib line out a few inches at a time until it's optimized.