Your computer's hard drive is used to store all datum: your operating system, your programs, your e-mails, your photos, your music, ... absolutely everything! First, let's forget names like disk and stuff and take a notepad: your have a notepad in which you constantly change the texts, you add some text, continue the ones you had started before, delete texts, etc. As you would guess, sometimes pages get filled and you have to skip some pages to continue your texts (and sure don't copy the first part of the text, it would take so much time!). It's easy while writing, but harder while reading (as you will have to skip pages -people that don't believe me may try the system out by taking a book, randomly arranging the pages, and trying to read). Your disk also has the same problem: it "randomly" stores data, and, as it gets filled, it becomes harder and harder to fetch your files!
A disk defragmentation utility takes the sectors and tracks (sorry, pages) on your disk (or on the notepad) to arrange them in the good order. Moreover, since your programs generally read certain files in a certain order (like Windows reading the exact same files in the exact same order while starting up or your e-mail client reading itself, your profile, its HTML interpreter and other component, your mailboxes and then connect) this utility also does a clever move: it orders the files in the order they are most likely to be read, so programs start faster...As a result, I would recommend you to run that tool in three cases: once you've installed your system, after doing some big changes on your computer or every month (or every two months, as you want). It's generally a good idea to leave your computer on its own while it's defragmenting the disk (the disk is busy, so won't have time to reply to your requests!). After having defragmented, it would be clever to reboot to system and maximize the "refreshment effect".
Are there cases where disk defragmentation is not necessary? Yes, there are! On SSD drives, which are hard disks which do not have rotating plates but rather pure electronic storage, disk defragmentation is not necessary: since SSDs do not have any mechanical parts, reading from them, even when the data is distributed all around, doesn't cause any delays (imaging it like a notebook where all pages are readable at the same time). Most operating systems will actually not even allow you to run disk defragmentation on an SSD drive.
Another important point: some people would claim that Linux or macOS don't need defragmentation, and some also claim that similarly NTFS doesn't require defragmentation. As you would see from the notepad example, that's totally false: there's a high chance for the files to be fragmented or for them not to be in the good order! Still, there's a reason for people to say such things: in ext4 or NTFS, there's an index for the files (for the notepad, it's like having a piece of paper where you've written which part of which text is on which page). This sure is faster than FAT (which has to seek page by page to collect a text), but you can easily show that defragmentation has a positive effect on most partitions we managed to invent (except for fixed file systems, like on CDs or DVDs).
And, the way to the disk defragmenter on Windows: Start - Programs - Accessories - System tools - Disk defragmenter. On Linux or macOS (and similar) you generally won't have a defragmentation utility included, so you'll have to download (or worse buy, therefore pay for) one.