I am not a techie, but in simple terms, bitstream is just how the signal is processed. They are also called 1 bit converters. It was Philips technology. Instead of a 24bit, 20bit, 16bit etc. data stream, the data stream is broken down into 1 bit pieces and them mixed/sampled up to whatever resolution the techonology could produce. Or put another way instead of a 20 bit data stream, you would have twenty 1 bit pieces of data. The upsampling / oversampling used was called "Continuous Calibration" instead of the 8X oversampling you find in many multibit DACs. It was at time the best way to reduce jitter at a reasonable cost.
Many CD players using bitstream had resolution equivalent to 20 bit. One of the best Philips chips was the TDA1305. You could search the model number to find players and more specs on it. Companys like Naim, Roksan Caspian, Cambridge Audio (their best CD6 and DACMagic), Rotel and others used this chip.
It is old technology, but not yet inferior if coupled with good electronics.