abs is a ternary expression.
put basically, it asks a true or false question, and returns the true part or false part based on the result
so in this calculation:
abs([Quantity] >= [breakALow] && [Quantity] <= [breakAHigh])?[Price] - [breakAAmount]:0)
the question is:
[Quantity] >= [breakALow] && [Quantity] <= [breakAHigh]
is the Quantity Greater than or equal to breakALow AND is the Quantity Less than or equal to breakAHigh
If true it returns:
[Price] - [breakAAmount]
if false it returns:
you enter the calculation on the calculations tab of the eCart object.