It means the size on the bottom row, for example 1.5 means that Control and Alt keys are x1.5 of regular x1 size key and inbetween them there is sometimes x1 windows/menu key and sometimes nothing. 1.25 means that Control, Alt and Windows keys are x1.25 size, this is more common on modern keyboards and it is a personal prefference on which you like, though I have to warn you that you will have harder time finding the desired key set you like with 1.5 boards.
Edit: Forgot to mention that 1.5 boards have x7 spacebar and 1.25 have x6.25.