Imagine that you moved into a new empty apartment. You buy stuff for the apartment, furniture, utensils, food, etc. But everytime you buy something, you move it all in the living room until the living room is full. Then you move new stuff into the kitchen, and when full, move new stuff into the bedroom. If you're hungry, you may have to go to the living searching for food and sometimes the bedroom. You might have to search for spices in the bathroom. Nothing is organized. When you defragment, you organize the apartment so that the food items are all in the kitchen, TV and electronics in the living room, etc. Next time you need to make food, everything will be in the kitchen, close to you; no searching around and no extra walking around to other rooms. You can imagine that each room in your apartment is a computer program. It'll run quicker if all the pieces of the program is next to each other, not spread out across your hard drive.