It's always hard to decide how to break into the circle of any natural science. In the beginning was. . . stardust? Starting there will take too long. Let's start with sand instead. Or rather, sediment, of which sand is a common part.
What is sediment? Conventionally speaking, sediment is any loose solid material that settles out of water. Like coffee grounds.
I'm looking down at the dry river bed of the Santa Fe River in the photograph below, where floods have washed down sediment from the foothills. As the water slows, the solid stony material it was carrying settles out to form the river bed.
The Greek word for broken is clast. Consequently, geologists often call this kind of sediment clastic sediment. Loose particles from the size of boulders to the finest clay, dropped out of water, air, or ice, are all examples of clastic sediment.