If someone is guileful, they are full of guile: they use clever but dishonest tricks to get what they want. It is the adjective made from that noun. You might read about a guileful smile, a guileful plan, or a guileful politician. The word is quite literary; in speech, people reach for sly, cunning, or two-faced instead. Though actually, it is rarer than its opposite, guileless — writers seem to need a word for 'no tricks' more often than a fancy word for 'full of tricks'. It is not a compliment. It says the cleverness is bent toward hiding the truth.