Haiku's Boot Loader can help when you experience hardware related problems or want to choose which Haiku installation to start, if you have more than one (maybe on an installation CD or USB stick).
It's also handy after you installed a software component that acts up and prevents you from booting the system to remove it again. The Disable user add-ons option that's mentioned below, will start Haiku without loading user installed components, e.g. a driver.

To enter the Boot Loader options, you have to press the SPACE BAR right at the beginning of the boot process. It's easy to miss so you best keep hitting the key until it shows up.

Once it's there, you're offered three menus:

Select boot volume		Choose which Haiku installation to start.
Select safe mode options		There are several options to try in case of hardware related trouble. When moving the selection bar to an option, a short explanation appears at the bottom of the screen. - Safe mode - Disable user add-ons - Disable IDE DMA - Use fail-safe video mode - Don't call the BIOS - Disable APM - Disable ACPI - Disable IO-APIC     [only available when IO-APIC is detected] - Disable LOCAL APIC     [only available when LOCAL APIC is detected] - Disable SMP     [only available when more than one CPU core is detected] - Enable serial debug output - Enable on screen debug output
Select fail-safe video mode		If you had to activate the option Use fail-safe video mode, you can set resolution and color depth.

After activating one or more options, you return to the main menu and continue booting, which presents you with this boot screen:

If everything works OK, one symbol after another quickly lights up.
The different symbols roughly correspond to these boot stages:

Atom		Initializing modules.
Disk + magnifier		Creating rootfs (/) and mounting devfs (/dev).
Plug-in card		Initializing device manager.
Boot disk		Mounting boot disk.
Chip		Loading CPU specific modules.
Folder		Final initialization of subsystems.
Rocket		Boot script starting the system.