what is sediments?

Answer 1

That which does not dissolve in water despite stirring and mixing the solution well, is a sediment.
Usually, we talk about fine rock gravel, sand, and grit particles that settle at the bottom of the water --in a mixture-- can be called Sediments.

Answer 2

Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid.