Erosion is the process that breaks things down.
In nature large objects are broken down to smaller objects.
As far as humans are concerned erosion is the breakdown of continents.
Mountains are rained on and become hills.
The left over pieces of the mountain are eroded to smaller pieces and goes down the side of the mountain.
Erosion in fact will always happen is a downhill direction.
In some places plants are the only thing holding soil together if the plants won't there erosion would be crazy.