It's "inputs" and "outputs" of an operation. The part before - is what needs to be on the top of the stack to be used. The part after - is what's placed on the top of the stack after it's over. The letters stand for data types (Builder is noted as b, and so on).

So in this particular case this notation means that the operation requires an instance of Builder data type to be on the top of the stack, it takes that Builder instance and returns a number (of bits already stored in that Builder), placing it on the top of the stack. Note that the Builder itself disappears from the stack in process.