The word subtle has two possible meanings.
1) Delicate or fine in texture, appearance, or quality; not coarse or rough. For example, a delicate lace is described as having a subtle pattern.
2) Not obvious or easily perceived; indirect or implicit. This meaning refers to something that is difficult to detect or notice because it is hidden or disguised in some way. An example of this use would be "The artist's subtle use of color created a sense of depth and movement."