You are skilled in something when you are really good at it. You might be naturally skilled at something, or you might gain skills by practicing.
The boy in the picture is a very skilled sailor.
Something is skinny if it is very thin, even to the point of being unhealthy.
Skipper is an informal word meaning the captain of a ship, or simply the person who is in control of the ship.
A skirt is piece of clothing that has a band around your waist but otherwise is just cloth below the band. Skirts go around both of your legs and not between them.
Skirts come in all lengths from very short, knee length like the one in the picture, or even down to your ankles. Usually only women and girls wear skirts.