عن أبي موسى الأشعري رضي الله عنه كَانَ إذا خَافَ قوماً، قالَ:«اللَّهُمَّ إنَّا نَجْعَلُكَ في نُحُورِهِمْ، وَنَعُوذُ بِكَ مِنْ شُرُورِهِمْ». [إسناده صحيح] - [رواه أبو داود والنسائي وأحمد] المزيــد ...
Abu Mūsa al-Ash‘ari (may Allah be pleased with him) reported: When he feared mischief from some people, he would say:"O Allah, We ask You to ward them off (from their throats), and we seek Your protection against their evil." Its chain of narrators is Sahih/Authentic. - [An-Nasaa’i]
The supplication: We ask You to be ahead of them, to push them away from us and to protect us from them.
He mentioned the 'Nahr', which is the neck or chest, because it is the quickest and strongest way to push someone away, given that the enemy approaches the fight with his chest forward. Or, perhaps it denotes optimism with the prospect of cutting their necks, i.e. killing them.
"And we seek Your protection from their evil", i.e. when Allah wards the enemy off on our behalf, He keeps us safe from their evil.
In other words, we ask You, O Allah, to stop their advance, avert their evil, and deal with them on our behalf.
They are only a few words, but if one says them with sincerity and truthfulness, then indeed Allah will grant him protection.