Can someone please explain how this works in the editor for me, thanks

It's a condition, right? In that case, it means "When the unit is converted", that is, if you have a trigger saying

Capture unit [At-at Blizzard]
Display Instructions: "We have captured an imperial walker".

If it's an effect (I think it's a condition, however), it would mean "turn over control of unit", but I guess that's the "Change ownership" effect.