Minimum return is a value that you pass into every conversion transaction. It serves as a protection mechanism for cases when the rate changes rapidly and allows you to "abort" the transaction without executing it.
Imagine a case in which you wish to convert 1 ETH and expect to receive 100 BNT in return. In such a case, you can set the minimum return to 90 BNT indicating that if the rate changes during the execution process to the point where you will receive less than 90 BNT as a result of the conversion, you no longer wish to execute the conversion and the transaction will fail.
The minimum return value must always be indicated as an integer in the format of the smallest available unit (Wei). The recommended amount is 2%.