Sediment is material carried by running water, waves and ocean currents. There are many other words meaning the same thing. Mud, silt, sand are all commonly used words. Geographers also use the word alluvium.

What is carried depends on how fast the water is moving. Waves, for example, crash on a beach most of the time, and can carry pebbles and even boulders with each wave. You can feel the beach material (sediment) moving if you stand in the wave zone (video below). On the other hand, rivers carry very little material normally, and that is why the rivers are clear. But when rivers are full, or when they flood, they pick material (sediment) from their beds and carry it along. At these times the water turns brown, and that is all of the mud being carried.

Many gold rush areas relied on the gold being washed naturally out of rocks and carried by rivers. Because gold is so heavy, the gold settled out into the river bed alluvium, or sediment. What the miners did was to wash out the river sediment looking for the gold. They called it placer mining.