It's a mode dedicated to listening on the CAN bus and logging all detected arbitration IDs.
Opening the help menu:
$ caringcaribou listener -h
-------------------
CARING CARIBOU v0.x
-------------------
Loaded module 'listener'
usage: caringcaribou listener [-h] [-r]
Passive listener module for CaringCaribou
optional arguments:
-h, --help show this help message and exit
-r, --reverse Reversed sorting of results
Example use:
$ caringcaribou listener
-------------------
CARING CARIBOU v0.x
-------------------
Loading module 'listener'
Running listener (press Ctrl+C to exit)
Last ID: 0x002 (2 unique arbitration IDs found)
(stop the listener with Ctrl+C)
Detected arbitration IDs:
Arb id 0x001 114 hits
Arb id 0x002 13 hits
In the above example, two arbitration IDs were detected. The message with ID 0x001 was seen on the CAN bus 114 times, while the one with ID 0x002 appeared 13 times.