I just pressed the power button and the volume up button in my Tab. I went into a screen called ODIN mode... It started downloading something... What is this?
This is the download firmware mode.
It can be used to upgrade official firmware via Samsung's own Odin software (or the 3rd party heimdall (also here)). It's also used to install a custom aftermarket firmware like CyanogenMod. Actually it allows more but installing firmware is the most common use.