Skillful is the American spelling of skilful. It means you are good at something because you have learned and practised — your hands or your mind know how. A skillful cook, a skillful player. In the UK and in most places that follow British spelling, the word is skilful (one l in the middle). In the US, skillful (two l's) is the normal form. Same meaning; only the spelling changes. You may still see skilful in some careful American writing, but US books and websites will usually show skillful.