What does it mean for a boat or ship to swing to anchor?

When a boat/ship/vessel wants to remain at the same spot, it either moors or anchors. Mooring involves attaching the boat to a buoy, which is itself attached to the ground, usually by a chain. Anchoring involves dropping an anchor carried aboard the vessel until it hits the ground and (hopefully) digs part of itself into the sand, or mud. The anchor is attached to the vessel by a chain or rope (often called a CABLE). In either case, the vessel is secured by a single thread to the ground and can move around as the tide and wind affect it. Thus it SWINGS to anchor. If we imagine a situation in which there is no wind, only tide, then every six hours (or thereabouts) the tide will reverse its direction and the vessel will turn through 180 degrees. Often the tide will have changed direction during each of the six hours (6:13 to be precise, but even that varies) and the direction in which the vessel is pointing will be constantly changing with the tidal movements. With wind, it just gets more complicated! The vessel is constantly SWINGING.