It basically means that something which appears to be a part of the background, such as music in movies which is solely for the viewer’s benefit, is actually part of the world of the characters and can be perceived by them.
I’m not sure if someone’s mentioned this before, but considering the mutts don’t seem to be autonomous in their decision making, it’s possible they’re not here for the group at all. The scout from a few strips back seemed to only register the wreckage, so the Topsiders most likely sent them on a salvage mission.
Depending on how single-minded mutts are in carrying out their tasks, it’s possible they’ll ignore everyone not directly in their way.
It basically means that something which appears to be a part of the background, such as music in movies which is solely for the viewer’s benefit, is actually part of the world of the characters and can be perceived by them.