Sedimentary describes things made from sediment — those tiny bits of rock, sand, and earth that sank and piled up, usually under water. The phrase you will meet again and again is sedimentary rock: rock that formed when those layers were pressed together over a very long time. Sandstone, limestone, and shale are the usual school examples. It is almost a classroom-and-geology word; people do not use it much in ordinary chat. Though actually, once you know it, you can look at a cliff of layered stone and say it looks sedimentary — stacked like pages, not cooked like volcanic rock, and not squeezed and changed like metamorphic rock. In geology talk it can also be used as a noun for that kind of rock, but the adjective is what learners meet first.