If you're skillful, you're very good at some particular thing. A skillful debater might have a successful career in politics, while a skillful baker could make the best brownies you've ever tasted.
Skillful people tend to be experts at something, like a skillful ping pong player or a skillful pianist. You can also use this adjective to mean "done in an expert way," so you could praise your sister's skillful snow fort building or your brother's skillful video gaming. The adjective skillful comes from skill, which is rooted in the Old Norse skil, "distinction, discernment, or adjustment."