It really has no informative signs on it just signs warning not climb or careful with your ball. It is in a Japanese park. I thought it is a water mound with its lots of pipes coming out it for aquifer to water the park.
As Jem Eripol points out, it is to prevent erosion / collapse of the earth above it. Likely the land was cleared and leveled to produce the park, and to prevent the hillside above it from filling in the space, the retaining wall was added. There is also the element of the designer thinking it "looked cool" -- that is a function in and of itself.
Hope that helped.