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 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 be 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, for example 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.