Fat is a specific substance. There are many different fats. Some you can eat, so you use them for cooking. Other fats are used in industry: grease makes a machine run better.
Fats are made of three different elements: oxygen, hydrogen and carbon. Some fats are solid. When they flow at room temperature, they are called oil. Grease is not soluble in water. If you pour it into water and the fat is lighter than water, a thin film forms on the surface of the water. Some fats are transparent, others are dark.
People also need to eat fat to survive. Among other things, the body needs it to build new cells. The cell membrane, which is the outermost layer of the cell, consists of fats. Without a cell membrane, the cytoplasm in the cell would leak, which would be fatal. Fat also contains a lot of energy. When you don't need this energy, for example through exercise, the body puts on the fat as a reserve. This makes us fat.
All animals that hibernate eat a reserve of fat in the summer. They live on this in winter and need the energy to warm up again when they wake up and get moving. In the past, people also ate fat to keep warm in winter. Fat insulates heat.