So good progress I think now

I am a bit confused about the power reading though. I am connected to the power device (stages crank left side on stages SB20 bike) and I am getting data, but it does not seem to correlate to what it should be, and its leaping all over the place. Looking at the device profile two elements are listed for power as below.
Byte 6 Instantaneous Power LSB
Byte 7 Instantaneous Power MSB
Am I supposed to read one of these, or both or add them? I don't understand what LSB and MSB mean.
I get readings but they don't seem quite right to me. This could be totally wrong as I am learning as I go, so any help would be gratefully received!
Thanks