I studied geography in secondary school and the word deposition sounds familiar .I had to do some little research though
Based on my research,When a river is fast flowing,it can transport sediments.If the river slows down,then it will no longer be able to transport sediments, and this materials will gradually begin to settle out of water.The settling of sediments is called deposition.
In other words,deposition is the addition of sentiments to landform.Landforms are weathered or eroded all the time and their pieces have to settle somewhere.
The setiments are be carried by wind or water or other forces and settle on another landform.
Deposition seem like a pretty simple concept but trust me,its not that simple.A variety of forces move sentiments in a patterned or messy ways.This can produce clear landforms like delta.It can also give us areas of plain sentiment like sandy desert.
The existence of this unique land features shows that deposition is not totally random.