fsboot is the boot command used by default on iBoot. It is what loads the kernel, background tasks and other modules.
Decompilation
Decompiled from iOS 9 from the iBoot source code leak.
static int do_fsboot(int argc, struct cmd_arg *argv) { if (argc >= 2 && !strcmp(argv[1].str, "help")) { printf("%s: load file from $boot-device on partition $boot-partition at path $boot-path\n", argv[0].str); return -1; } return mount_and_boot_system(); }