A meander is a curve in a river. Meanders form a snake-like pattern as the river flows across a fairly flat valley floor.
The river erodes sediment on the outside of the curves and drops it on the inside of curves. This is because the water moves fastest on the outside of a turn. Slip off slopes are formed on the inside of the bend from deposition and river cliffs are formed on the outside of the bend from erosion. Eventually meanders turn into ox-bow lakes when two outside bends erode together making a shorter route for the water.
The position of the curves changes over time.