What is under keel clearance and why is it important?

Under keel clearance looks like simple subtraction, charted depth minus draught, until you account for everything that changes on both sides of that equation between the chart being surveyed and the ship actually passing over that spot, which is why UKC calculation is one of the more detailed parts of passage planning in shallow water.

Charted depth is referenced to chart datum, a low-water reference, not the depth at the moment of transit, so the height of tide predicted for the time the ship will be there has to be added to get the actual expected depth. Tidal predictions carry some uncertainty, particularly where tidal behaviour is complex or poorly surveyed, and actual conditions, strong onshore or offshore wind, low barometric pressure, can produce a surge that pushes the real water level away from prediction, something navigators in tidally sensitive areas learn to watch for.

On the ship's side, static draught is only the starting point. Squat, the increase in effective draught caused by a ship moving through shallow water, where restricted flow beneath the hull accelerates and lowers pressure, pulling the ship down and often changing trim, can add a metre or more at speed on a large, fully loaded ship in a narrow channel, and squat increases sharply with speed, which is exactly why ships slow down on shallow approaches, not only for manoeuvring control but specifically to limit it. Heel during a turn increases draught at the outboard bilge; rolling and pitching in a seaway can transiently increase draught at the ends; and water density affects draught directly, since a ship drawing a given draught in seawater draws more in the lower-density fresh or brackish water often found near river mouths.

Passage planning works out UKC at every critical point along the route, applying all these factors plus a prudent margin to cover residual uncertainty in the survey, the tide, and the ship's actual condition, and where the margin comes out tight, the passage is timed to the best available tidal window with speed reduced to control squat. Misjudging the tide, ignoring squat in a laden ship entering a shallow channel, or working from an out-of-date chart with unsurveyed shoaling, is a recurring cause of grounding, which is why UKC is a checked, mandatory element of the passage plan rather than a rule of thumb.