In the wikipedia article for the song Iris it says there is an:

unusual guitar tuning in which most of the strings are tuned to D, lending the guitar a chorus-like effect.

In a follow up article follow up article it says:

Perhaps the most famous example of this is the B-D-D-D-D-D tuning of his signature hit "Iris"

Can somebody explain this to me? If one were to make every string of a guitar play D, wouldn't she have to completely re-learn how to play the instrument?

I have never played guitar so I do not know what "tuning" such as this as, but, to use another example, if every Piano player tuned her piano a different way, it would be impossible to play on any normal Piano.

Can somebody briefly explain how "tuning" a guitar works and why the B-D-D-D... tuning is different?