There is an area on the harddrive called the partition table. It records where each filesystem on the harddrive begins and ends and what type of filesystem it is. With fdisk you can change the number and kinds of partitions and their starting and ending places, but you cannot move the existing data around so changing the table with fdisk can cause you to lose data. This is not important if you do not care about existing data. This is less imprtant if you have a backup. This is suicidal if you have imprtant data on the drive and change the starting point of a filesystem and allow the filesystems to be written/deleted/formatted.
When fdisk starts, it reads the partition table. You can edit the table in memory, but, until you tell fdisk to write it back to disk, nothing irreversible happens.
One can use the anaolgy of a book. The partition table is like the table of contents. If you change the table to say Chapter 3 starts on page 47 instead of 23 and the reader needs to use the table, the book is useless unless you edit the book to adjust Chapter 3 in the book and the table both. Otherwise, one needs to rewrite the book.
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