The calculator can work with numbers written in three bases: decimal (the usual), binary, and hexadecimal. The 0b command indicates that a number is written in binary:
:0b101 5 :0b100000 32
When written in binary, numbers are expressed as signed 32-bit integers, which means that only the integers between -231 and 231-1 can be expressed in binary. With other binary-related commands, numbers are simply truncated to fit in this range. Not so with 0b: if you enter more than 32 binary digits after the 0b, the result is a domain error.
Even if the calculator is in binary mode, you still have to write 0b for an integer to be interpreted as binary: binary mode only affects output. If the calculator is in decimal mode, which is the default, you have to use ▶Bin to get output in binary.
260 - Domain error happens when 0b is used with more than 32 binary digits after it.