The code is essentially converting each character of the binary string biString into an integer using a radix of 2, which is why the 2 is written after biString[i] in the code.
The radix is an integer between 2 and 36 that represents the base in mathematical numeral systems.
If the radix is 2, the string is interpreted as a binary number, and if the radix is 16, the string is interpreted as a hexadecimal number. The radix parameter is optional, and if it is not specified, the function assumes a default value of 10 Understanding the Radix