Mantras work whether you know their meaning or not. Mantras are vibrations, that is why their proper intonation is important. If I am tone deaf but know all the words to a song, your ears will still not find my singing appealing.
When a mantra is given by an enlightened person, they put some power into the transmission to you when you hear it. Many people who have received a mantra from an enlightened person and were ready for spiritual practices, remember that instance and the exact intonation for the rest of their lives. Such is the power from an enlightened person. The mantra that is given is the vibration that is best suited to match your own vibrations and bring your personal vibrations into harmony with the divine vibration.
There is a story in the Taittiriya Samhita (II. iv. 12.) showing the adverse effects of not intoning a mantra correctly. A god, Tvashtri, in pronouncing the word Indra-satru, he accented the first instead of the last syllable. In short, instead of taking away power from Indra, he gave Indra more power.
There is also the story of Ratnakara, the highwayman who was given the mantra 'Rama' to say by Narada. Such was his tamasic nature of his mind at the time that he could not remember how to pronounce it correctly. So he started to say 'mara' (which means death in Bengali) instead of 'Rama'. But he persevered and kept on chanting it continually, until it became ma-rama-rama-ra..... and thus from having a sincere heart and perseverance, was transformed into the sage Valmiki, the author of the Ramayana.
Sincerity of heart and perseverance and the Lord Himself will clear your path.