In a short form “Trekking is an act of walking for pleasure”.
Trekking is walking along the mountain/ Hill / Himalayan  trails for pleasure or sports. It provides an opportunity to explore the villages, villagers, valleys, mountains and its interior. So trekking means foot travel on mountain trails.

The Wikipedia’s definition of trekking: “Trekking is a form of walking, undertaken with the specific purpose of exploring and enjoying the scenery. It usually takes place on trails in areas of relatively unspoiled wilderness. “