The word mantra means "that which delivers one from the material world." Only the dvijas (the brāhmaṇas) and the devas (the demigods) can be delivered from material existence by the instructions of the Supreme Personality of Godhead. Whatever is spoken by the Supreme Personality of Godhead is a mantra and is suitable for delivering the conditioned souls from mental speculation.
(Srimad Bhagavatam----8:6:15----purport).

Who is only speaking about Kṛṣṇa, the Supreme Truth, he is called satya-vāṅ. He does not speak anything... Kīrtanīyaḥ sadā hariḥ (Cc. Ādi 17.31). He is simply glorifying. If one's life is dedicated to preach about Kṛṣṇa, then he is also satya-vāṅ. He does not speak anything else. So our, these Kṛṣṇa conscious members, they are simply trained up to go and speak about Kṛṣṇa. Therefore they are satya-vāṅ. They do not speak anything else. They do not waste their time by speaking anything which has no relation with Kṛṣṇa. That is satya-vāṅ. Satya-vāṅ mantravit. Mantravit means Vit means one who knows, and mantra means Vedic hymns. Mantravit.
(Lecture on Srimad Bhagavatam----6:1:56-57).

So mantra means... Namaḥ. Nama means surrender, to become submissive. Namanta eva. Namanta eva. That is the qualification. Namaḥ. When we utter this word namaḥ, means "I surrender.
(Initiation Lecture).

Mantra means... The Sanskrit word mantra, man means mind, and tra means deliverance. Mantra means that which delivers you from the mental concoction or hovering on the mental plane. Every one of us is hovering on the mental plane. By mind we are creating so many things: "I shall be happy in this way. I shall be happy in this way. I shall be happy in that way." But mantra means when you are delivered from that concoction, speculative way of your mind, and you come to the transcendental platform. That is the effect of mantra. So this mantra, this Hare Kṛṣṇa mantra, is called the mahā-mantra. Mahā-mantra means the greatest of all mantras.
(General Lecture).