The output of a single phase detector is a one-bit signal. The author intends to take several phase detectors which produce one-bit signals and add their outputs, resulting in a single signal which has multiple bits. For example, if there are eight phase detectors which all have 1-bit outputs, the result is a single 3-bit signal whose value is simply the binary sum of the phase detectors' outputs. The author described this operation as "boxcar filtering" and "decimation," but it's much simpler than that.