The spear in the daylight is easily avoided. The arrow in the night is difficult to defend against.
-The Ravages of Time
The meaning here is clear: Underhanded methods are the most difficult things to be aware of. While threats can be easily seen if they are huge, the most dangerous ones are the ones that remain hidden. Those that bide their time, those that hide their malice and hate; those are the ones to fear, while those who openly express their views can be easily taken care of.