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.
Tiberius Sempronius Gracchus (163/162 BC- 133 BC) was an important Roman Tribune of the plebs. He proposed a reform called the Lex Sempronia Agraria. This was meant to reduce the number of poor and homeless people by allowing the rich to have only a certain amount of land. The rest of the land would be given to homeless veterans of the Roman military.