National Catholic Register, Matt D’Antuono: St. Augustine explains that the word prayer does not simply mean speaking to God, though that is certainly one way to pray. Prayer, in its broadest sense, means desire:
What else is intended by the words of the apostle: ‘Pray without ceasing,’ than, ‘Desire without intermission, from Him who alone can give it, a happy life, which no life can be but that which is eternal?’ … We ought by cherishing holy desire to pray without ceasing. For to spend a long time in prayer is not, as some think, the same thing as to pray ‘with much speaking.’ Multiplied words are one thing, long-continued warmth of desire is another.
This passage also cleared up for me a question about which I had always wondered: how am I supposed to pray without ceasing? But since true prayer is desire, words are not necessary and sometimes even unproductive:
To use much speaking in prayer is to employ a superfluity of words in asking a necessary thing; but to prolong prayer is to have the heart throbbing with continued pious emotion towards Him to whom we pray. For in most cases prayer consists more in groaning than in speaking, in tears rather than in words.
Read in full here.