Moulin (pronounced moo-lan) is the French word for mill. It is also the English word for what is sometimes also known as a glacier mill (completely different from glacial till, by the way). These are places where flowing water enters the glacier, forming englacial or subglacial watercourses. These are often large streams of meltwater that are dropping suddenly into a crevasse, or swirling violently into a vertical shaft.
Glaciers and ice sheets often melt a bit in the warmer months, releasing water from their surfaces. This water may flow along the surface, or collect in pools and lakes. Water sitting on top of a glacier will absorb more heat from the sun than will ice (ice reflects a lot of solar radiation), and so a stagnant pool may warm up and eat through the ice until they find an outlet. But more often moulins are formed where flowing water finds a crack or crevasse in the ice sheet, and starts to erode a larger watercourse. The warmer meltwater can eat networks of tunnels in and under the ice, sometimes creating giant caverns.
Moulins can be tens of meters across, and go down hundreds of meters before they reach ground level. A established moulin will normally take on the shape of a cylindrical chute, and when dry it looks like a giant wormhole, diving deep into the glacier. When the water hits dirt, it may do so with enough force to drill out a 'kettle', AKA a moulin pothole. Alternatively, it may quickly lose momentum and drop sand and rocks that were being carried by the water, leaving a mound called a moulin kame. Other formations that may be left along the course of an englacial stream are eskers, terraces, and deltas.
All this water running under the ice can help lubricate the iceflow, making the glacier move along faster. This is currently happening in the Arctic icesheets, with the result that more ice is being pushed out into the ocean and melting.