How does F/A-18 autotrack work? This is about the alleged "UFO" video. However, I'm not interested in speculation about what exactly the aircraft being tracked in the video is.

What is "autotrack" in an F/A-18 Super Hornet? Does it involve tracking by the radar, infrared, or digital interpretation of the video stream by a computer?

I've provided a link to the video. This shows the aircraft (the alleged "UFO") being "tracked" by what the pilot claims to be "autotrack":


                
            

As a bonus question, what is the other pilot asking about? He asks "Did you box a moving target?", what does it mean to "box" a moving target? Does it refer to radar tracking?
Autotrack means the FLIR (Forward Looking InfraRed) sensor attempts to track whatever object falls inside the tracking area. It means whatever the pilot was trying to keep up with was producing emissions in the infrared (heat) and the system was automatically tracking it.

Boxing a moving target is having the tracking system detect and track a target you have selected. In this case, the other pilot was asking if he had selected the target manually (boxing) or if the system detected and autotracked the target on its own.