In a way that is not easy to notice or see, in a quiet or gentle way.
When something is done subtly, it is done in a way that is not obvious or loud.
Examples:
She smiled subtly so no one would see.
He hinted at the answer subtly.
The cat moved subtly through the grass.
The joke was so subtly told that no one laughed.
The magician subtly made the coin disappear.
The treasure was hidden subtly under the old tree.