What is a cable attenuator ?
Generally, line attenuators are designed to reduce the signal level to bring it down to specs without distorting the signal waveform. Attenuating the downstream will decrease the signal level/dbmV, while attenuating the upstream frequencies causes your equipment to increase its transmit power to communicate.
Cable modems work best if the downstream signal level is within +/-15dbmV, and the upstream power is between 37dbmV and 55dbmV. If the signal is too strong on the downstream, or too low on the upstream, a tech may need to install an attenuator. There are different types of attenuators, as follows:
Attenuator - a plain attenuator will weaken (attenuate) all frequencies, reducing the downstream signal strength, and causing the upstream power to increase (the modem needs to "talk" louder).
Forward path attenuator - weakens (attenuates) the high frequencies used for downstream only, without affecting the upstream frequencies.
Return path attenuator - weakens (attenuates) only the low frequencies used for upstream. This is best installed far from the cable modem upstream, i.e. outside of your residence at the tap. When you attenuate the upstream frequencies, the cable modem is forced by the CMTS to increase its power, to "speak louder".
See also: What cable modem signal levels are considered good?