Subtle (the "b" is silent, by the way) means slight or barely noticeable. For instance, if you move something subtly, you move it only a little bit. If something is subtly different, it's only a little different, in a way that is hard to notice. Ever go into a room and you know something is different but you're not sure what? Someone has made a subtle change to the room. They didn't paint it, they didn't redecorate it - those are big changes. A subtle change might be moving a photograph from a table to a shelf. A subtle smile would be a very small smile. You could also say that a person is subtle, which would probably mean that they think/behave in a way that is difficult to understand, maybe because they have goals that you are unaware of. This is not a common use of the word though, although you might see it in literature. Answer from Deleted User on reddit.com