Ok , N00bfriendly answer follows :
The wet dream of most console and pc hackers since day one is to completely remove the operating system in a machine and write their own operating system from scratch to communicate with all the hardware . After this is achieved you can write your own software in whatever programming language you installed for your custom operating system to read , hence taking FULL control over the machine without breaking any copyrighted code and without having to be forced using the programming language from the manufacturer.
This has afaik never been achieved fully in any console scene EVER .
There has been many failed attempts thru the years . the last one I can think of was Mathieulh´s Utopia project for psp .. The buzz was high for a while , but it died down to nothing.
Enter Bootmii !!
Finally Team twiizers have actually pulled it off and made a complete system replacement operating system . ! The achievement is HUGE ! And will go down in history .
What will this do for a normal user ?
Bootmii is among other things a boot replacer . Much like preloader , BUT it lets you do a complete nand backup or restore even if your wii is real messed up.
(as long as you have made a nand backup from within bootmii in advance)
Bootmii also has its own operating system called "mini" . This is written in a different programming language than usually used on wii homebrew (devkitpro\libogc) , so currently all developers will have to rewrite their apps to make them run from there.
This means that until some easy way to convert programs from one language to another is developed , all devs must rewrite everything from scratch so for the normal user Bootmii is nothing but a real fancy brickprotection the next few days.
Still , just be patient and bootmii will eventually turn into your new main system menu .
Hope that helps the n00bs