The root filesystem (`/`) contains the kernel and utilities needed to boot a system.
The root filesystem is the filesystem containing the root directory
Other filesystems can subsequently be attached as subtrees of this. The init program is loaded from the root filesystem and may kickstarts a process to mount other filesystems.
Traditionally the root filesystem is the first to be mounted. On Linux, it is common to first mount another filesystem from an image in memory, either initramfs or initrd. This initial filesystem contains additional software which may be needed to access the root filesystem, for example if the device containing the root filesystem requires additional drivers, or if the root filesystem is encrypted and the user must enter a passphrase to decrypt it. Once the root filesystem is accessible, it is mounted and the normal boot process starts with init.