A heart beat is a single pulsation of the heart. Heart beats are what what keep us alive. But without that one single heart beat, that one beat in isolation, there would be no heart beats thus rendering us without life.
To say someone is your heart beat is to say that they are the beat that keeps your spirit alive, your soul alive, your joy alive, you happiness alive and your reason for loving alive. Without them, your heart may physically beat but your spirit, soul, joy and happiness would be without a beat. Their presence in your life is what keeps the smile on your face, the joy in your day and the shine in your eyes.
Your heart beat is the person whose smile keeps you smiling, whose company keeps you laughing with glee. Everything about this him is perfect. Their presence brings light into your life and makes everything crystal clear. The mist of loneliness disappears and is replace by the veil of affection. He can make your heart race and slowly beat at the same time. Because he is the beat of your heart, all you can do is think about him, care for him so that he continues to beat in delight. Your heart beat is the beat that keeps you breathing.
Only He is my heart beat :)