Dear friends, at the outset, let's just humbly echo the prayer of this dear saint of God - Oh Lord, Let the words of my mouth and the meditation of my heart be acceptable in your sight.
Meditation is what we think - what we dwell upon in our hearts. Meditation helps us to be in contact with the spirit world. We are created with capability to allow our minds to communicate with the Lord God creator as well as evil forces. (As a side note, herein is the danger of 'other' forms of meditation like yogic, buddhist, etc - where you bare your mind in meditation and open your mind to intrusion from evil forces. See the testimony of a former new-ager mystic who understood this and turned to Christ http://www.simonarich.com/about-simona-rich/).
Our mind is very precious to God - He desires a deep fellowship with the human race which He has created in His image & likeness. One can't fellowship with a lower creation (for example, however much a person likes his pet dog, the dog will remain a dog and man will remain a man and the dog can never understand the mind of the higher being.) Contrast this with what Paul says in 1 Cor 2:16 For who has known the mind of the Lord ? that he may instruct him? But we have the mind of Christ. A born again person has the ability to understand things of God & to have His mind in a sense. And I thank God for Him opening this way to deep fellowship with Him, Yea, even to the extent that we are now His sons! (John 1:12)
We are the temple of the Living God - and our mind is the sacred place, so it's so essential to keep it Holy & Blameless. The most valuable real-estate in the world is your mind. (The lord estimates a human soul to be more valuable than the full value of the created earth!).
At this juncture, the prayer of this Godly man in Psalm 19:14 comes to our mind. If ever we've needed it more than ever before, it is NOW. Dear child of God, make it a point to study the thoughts of your mind to see what is occupying your mind for most of the time. List them - are these thoughts of evil origin, worldly origin, fleshly origin, thoughts of harm or thoughts of the holy spirit. Let not satan and his world system occupy the precious space in the temple of the living God.
Meditation is dwelling on the thoughts
You are what you think. Proverbs 23:7 says, “As he thinketh in his heart so is he.” . (If you see the portion in Ezekiel 8:8-12, it is a picture of the Lord seeing into the recesses of people’s heart.) Proverbs 24:9 reads, “The thought of foolishness is sin...” In Matthew 5:28, Jesus said that whoever looks at a woman with lust has already committed adultery with her in his heart.
Beloved, A holy inner life is the only thing that pleases God. 1 Samuel 16:7 (The LORD does not look at the things man looks at. Man looks at the outward appearance, but the LORD looks at the heart) Praise the Lord for His great power, love and grace to transform our meditations. Ephesians 4:22-24 (4:22) You were taught, with regard to your former way of life, to put off your old self, which is being corrupted by its deceitful desires; (4:23) to be made new in the attitude of your minds; (4:24) and to put on the new self, created to be like God in true righteousness and holiness.
Dear Child of God, may we be gripped by this holy desire to have a mind that's renewed that meditates on the things of God always.