It's a poetic and metaphorical way of expressing a strong emotional connection, often used in the context of romantic love. It signifies that the person being addressed continues to hold a central and irreplaceable place in the speaker's heart, even if they are no longer physically present.
Here's a possible interpretation:
"Even though we may be apart, you continue to be the very core of my being, the source of my emotions."
It conveys a sense of deep affection, longing, and an enduring emotional bond.