BG_MAP_BASE Help needed

[Prometheus]

Please, can somebody explain me what BG_MAP_BASE stands for?
I've made a simple test with two background (1 for the top screen and 1 for the bottom screen) and 1 console to print out text.
At first it didn't work, then I've found in this forum that somebody suggested to put BG_MAP_BASE(2) and it worked.
But I really can't understand why I have to put "2" as a value for BG_MAP_BASE.
Please, help me.
Thanks in advance.

[coreyh2]

http://dev-scene.com/NDS/Tutorials_Day_4

[vuurrobin]

and:

http://libnds.devkitpro.org/a00062.html

#define BG_MAP_BASE(base) ((base) << MAP_BASE_SHIFT)
Macro to set the map base in background control.