Global Listening allows BlueConic to pick up general visitor information. It picks up technical information like the browser name or the device, but also general statistical information like the number of visits or the average time of a visit. See What kind of information does BlueConic collect? for details on profile properties populated by the global listener.
Installing the Global Listener plugin is enough to activate it on all channels. No additional configuration is required. Note that even though the Global Listener will not be listed under "Profiles > Listeners", it will still be active.
Global listeners can be added to Objectives, allowing for privacy management of the information that is being picked up. The global listener will only update a profile if the visitor consented to at least one of the objectives that it is linked to. The global listeners are always executed, but properties that are being set are only persisted when consent has been given. Properties that are set can always be used in segments on the page even if they are not persisted.