Shri Krishna tells Arjuna that we have to attach our mind to God, and meditating is a very effective way of doing that.
Repeatedly thinking about something in a very deep way, in fact, is meditation. Our mind has the capacity to take on the qualities of the person or thing that we’re thinking deeply about.
When we meditate on someone in a loving positive way, our mind takes on their qualities.
So Shri Krishna says, “Mat-chittaha… Arjun, think of Me.”
He adds “mat-paraha,” meaning to love Him, to feel that He is my everything.
When we attach our mind to Him through meditation, we get those good qualities automatically, and the negative qualities like anger, jealousy, greed, and restlessness reduce.
“Upaviśhyāsane yuñjyād yogam ātma-viśhuddhaye” — our mind gets purified because He is pure and we are attaching our mind to Him.
Yoga means to join, and a yogi is one who practices joining the mind with God. Krishna did not say “clear your mind and think of nothing.” He said, wherever your mind goes, put Me there.
Jagadguru Shri Kripalu Ji Maharaj further elucidated these concepts and called it Roop Dhyan. Roop means the form, because if you’re going to think of someone, then you’ll think of their form.
Arjun asked whether to think of Krishna as formless or in His personal form, and Krishna said it’s very difficult to think of the formless aspect — think of Me in My personal form.
Roop Dhyan means thinking of Krishna’s form — His eyes, His hair, His face, what He is wearing, how He moves — and this is how we engross our mind in meditation, according to how Shri Krishna told us in the Gita.