Weathering and erosion are often confused:
For example, a limestone cliff may be weathered by freeze-thaw, a type of physical weathering. This means that rock in the cliff becomes broken into smaller pieces.
Erosion happens when these pieces of rock fall away down the cliff.
Rivers and streams can move pieces of rock. This is called transport. Fast-flowing rivers can transport large rocks, but slow-moving rivers can only transport tiny pieces of rock.
As the pieces of rock are carried along by the water, they bash against each other and the river bed. They gradually wear away because of this. They become smaller and more rounded.