Concepts can be approximate in at least two ways.
On one hand, a concept may approximate another more definite but incompletely known concept. This situation is prevalent with natural kinds . Lemons are a definite species, but no-one knows all about them. In particular, a child may be barely able to tell lemons from other yellow fruit but nevertheless has a concept of lemon that may be adequate to find the lemons in the supermarket. The child can improve its concept of lemon by learning more.