Terrain relates to the horizontal and vertical aspects of the land’s surface. There are many types of terrain of which the main ones are the plateau, mountain, plain, and valleys. Other terrain types include open, tundra, oasis, etc. (see here for more details).
Terrain is a derivative of the word “terra” meaning earth. Factors such as the slope, elevation, and orientation of the land are all features used to describe the different terrains. The terrain of land can also affect the weather pattern and climate of the area. For example, two areas that are geographically close to each other may differ entirely in precipitation levels or timing due to elevation differences.
To determine its suitability for human use/settlement.
To understand environmental quality in terms of agriculture and hydrology. For example, drainage, water movement, etc.
Terrains affect aviation especially for aircraft which have low-flying routes and altitudes of airports.