Answer:
Terrace farming is a soil-erosion prevention measure utilized by the Ifugaos in the ever-famous Banaue Rice Terraces.
A little bit of physics analysis for this matter. Gravitational pull is always downward. A certain element like water, coming from the top of the mountain, will definitely have an enormous momentum going down. As the water goes downwards, it accumulates a lot of water. So the force it produced is so enormous that can erode the soil whenever it touches it.
To counter this scenario, you have to build a neutral ground, a step in the form of a terrace, to slow down or eradicate the momentum going downhill by the water. On every terrace, the water will just spill over another terrace until it reaches the lowest terrace down the mountain.