/private/etc/fstab

fstab controls the read/write access of the root and media partitions. A barebones jailbreak can be put in place simply by modifying this file to grant full read/write access to the user.

Modification

The original fstab file, before a jailbreak, looks something like this:

/dev/disk0s1 / hfs ro 0 1

/dev/disk0s2 /private/var hfs rw,nosuid,nodev 0 2

Notice /dev/disk0s2 has /private/var mounted as read/write. This directory is generally where music/photos/videos/etc are stored. This directory can easily be accessed (without FULL access) with a program like PhoneDisk or DiskAid. iTunes has full and complete access to this directory, for obvious reasons. This is proven with the afc service, which iTunes uses to transfer music/videos/etc.

Jailbreak

Now, in order to get at least a barebones jailbreak going, we need to change

/dev/disk0s1 / hfs ro 0 1

to

/dev/disk0s1 / hfs rw 0 1.

This is the root partition, which generally has about ~800MB storage. This is also where Cydia installs applications. Once we have full read/write access to this partition, we can use the afc2 service to actually use our full access over USB. Most modern jailbreak tools like limera1n and greenpois0n install the afc2 service automatically, mostly for their own sake.