The PM52 Headphone Tap is for connecting headphones to a speaker line. The unit takes a speaker level signal and allows the user to "tap in" with a pair of headphones. It's just like using a stage monitor, but using headphones instead of a heavy speaker.
The 1/4" TRS phono Mono/Stereo loop jacks are for cascading more boxes in series, or connecting to a monitor speaker. There are two headphone jacks and a volume control. The PM52 uses the speaker line signal for power, so it requires no batteries or external power.