What do the debug arrows on NavmeshAgents mean?

I have a handful of NavmeshAgents in my scene that sometimes seem to "bounce" back from their destination. Once they get into this state they'll just keep doing that until their target changes.

When this happens, if I switch to the Scene View in the debugger, a black arrow will be drawn pointing away from their target which will sort of rubber-band as they move away. Then they'll start moving to their target again, and repeat. Turning gizmos off on NavmeshAgents will stop drawing these arrows.

I've googled and searched these answers to no avail. I'm sure there's an explanation of the things it draws somewhere, but hell if I can find anything.