The bigsmall byte is a way of storing two alternate units in one byte (like minutes and hours for example).

The most significant bit of the byte is used to determine whether the data is in “big units” or “small units”. 
So with the minutes and hours example, if the most significant bit is a 0, the byte is read by the device as minutes, but if it is a 1, then it is read as hours.

For example: 10000001 would be 1 hour 00000001 would be 1 minute