The standard scale on which most songs are based, uses 7 notes and in the 'key of C' these are the 7 white notes on the keyboard. It is possible to play any song starting from any notes on the keyboard, each different starting point being 'in a different musical key', with the key of 'C' being the easiest, as you usually don't need to play any black notes or learn rules about which  black notes replace white white notes in the standard scale. 

Most students never need to play in any key apart from C unless they are playing to accompany singing. Most electronic keyboards have a transpose button which enables you to still playing the key of C but have the music sound in any musical key you wish. 

If you are playing a keyboard which doesn't have a transpose button eg an acoustic piano, and you are accompanying someone singing, then you may need to play in keys other than C. The DecPlay system enables you to do this by instantly transposing to any key using the same song sheets. You simply change the starting point of your scale to the new key. Eg to play in the key of G, the number system 1 to 7 starts on G and you learn the sharps or flats in the key signature as usual eg note 7 of the scale is an f#.