If something is supersubtle, it is very hard to notice or understand.
Supersubtle things are often very clever or hard to understand.
Examples:
Her smile was supersubtle, but it made me happy.
The supersubtle hint in the story was hard to find.
The supersubtle flavor in the soup was hard to describe.
The supersubtle joke made everyone laugh.
The supersubtle change in the painting made it much better.
The supersubtle plan was very hard to understand.