What actually is a mantra

Mantra repetition simply means  repeating a sentence or group of words that have a phonetic significance, that's it.

Mantra  is intrinsically related to sound. Mantra is sound, and sound is reverberating in everything in this universe. When water flows, the gurgling sound it makes, is mantra. When wind blows through the trees, the  rustling sound it produces is mantra.  When we walk on the earth, our footsteps produce sound, and that too is mantra. Within  human beings  there is a self-born, indestructible sound which repeats itself constantly, along with our breathing and this sound is also a mantra. 

            Sound has enormous power, in fact it has the power to create an entire universe. It is written that God originally manifested as sound. 

(In the beginning was the Word, and the Word was with God, and the Word was God.  New Testament, John1:1-2 )

According to ancient Indian belief  in the beginning there was sound, which  reverberated as Om and from that sound everything came into existence. 

Even modern scientist are beginning to recognize as our ancient sages did,  that there exists a vibration which reverberates ceaselessly throughout the cosmos.  

            When letters and syllables come together, they form words. Both our spiritual and our mundane life are possible only because of words; without language, we can not carry out any of our activities. Each word we use has its own power and produces it own reaction. A mantra is no ordinary combination of letters and syllables, but a living force. The name of God is not different from God. Mantra has been called the sound-body of God: It is God in the form of sound. 

In  Bhagavad Gita Lord Krishna says, 

         " Among rituals, I am the ritual of mantra repetition." 

By this he means that while other techniques are means of attaining Him, mantra is his very being. That is why it is so easy to experience God by repeating the mantra. Mantra meditation is the repetition of a group of words which create  sound vibrations that awake the love of God in our hearts and mind.