Compare your computer for a while with the place where you live. Imagine you don't clean that place for days or even months. After a while it would be complete chaos. Finding something in such a mess takes time. It's just the same with your computer. The files you work with are regularly opened, updated and saved. After some time these files are spread all over the hard disk in tiny fragments. This slows down your computer. That's where "Defragmentation" comes in. Defragmenting the hard disk will put all the different fragments together in one place which allows the computer to save time. Compare it with cleaning up your place.