This is generally to remind the listener that this sentence has other meanings, not just superficial simplicity.
Deep meaning means profound meaning. Deep: deep, meaning: meaning, meaning.
It's mysterious and reasonable.
The mystery here refers to:
1. Providence; Secret. For example, in A Dream of Red Mansions, for the first time, "This is a mystery, which should not be revealed beforehand."
2. profound and subtle principles. For example, the third verse of Zhang's Four-character Poem of Taoism says, "The golden furnace inherits the Tao, and the jade butterfly reveals the mystery."
3. A clever plan. For example, Liu Zongyuan's "Sword Gate Inscription": "Swear, mystery in hand, life points, Chen is a corner."
4. refers to a conspiracy. For example, the twenty-ninth chapter of "Rebirth": "Why it hurts the sky is a mystery."