The ordinary meaning of nap is to doze off during the day. The secondary meaning is to be offguard. Hence the phrase Catch them napping . Flying by the nap of the earth refers to the practice of flying low to the earth (undetected by radar) to catch the enemy by surprise (or napping).
the tribunes The Tribunes were elected representatives that protected the plebeians from mistreatment. The Tribunes attended meetings of the consuls or Senate in an attempt to bring justice to the plebeians.
The Plebeian Tribune is also known as the tribune of the people or the tribune of the plebs. The plebeian tribune had no military function but was strictly a powerful political office. The tribune had the power to help the people, a function called ius auxilii. The body of the plebeian was sacrosanct.