It's a very romantic and poetic phrase that expresses the idea that the person being addressed is the only thing that matters to the speaker. It's a strong statement of love, affection, or admiration.
Here are some ways it can be interpreted:
* "You are all that I need."
* "You are the only thing that makes me happy."
* "I only want you."
* "You are my everything."
It's important to note that the exact meaning can vary depending on the context and the speaker's tone.