OK, lets break this down...

DecimalByte=(BCDByte>>4)*10+(BCDByte & $0F)

The calculations in the brackets will be perfomed first, and the results stored in a PICBasic variable behind the scenes. BCDByte is unchanged - just as well because we refer to it twice, once to access the top four bits, and once to access the bottom four bits.

Thereafter, PBP follows mathematical rules of priority... ie Multiplication and Division are performed before addition ans subtraction. Because items in brackets are always calculated from the centre out, and each set of brackets is it's own self-contained world, the result of the formula below, will be the same as that above.

DecimalByte=(BCDByte & $0F)+((BCDByte>>4)*10)

However, you do have an extra set of brackets there... but PBP actually resolved the formula without any additional penalty. Might not be so lucky on another occasion.