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 position of the curves changes over time.
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. Water does this because it tries to find the path with the weakest resistance. 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.
Meanders were named for a river in Anatolia.