An opening at one or both sides of a territory on the edge of the board, with opponent stones positioned so as to be able to slide or jump in. Weaker players tend to underestimate the negative value of an open skirt, especially since it's hard to find the right time to close it off, such moves usually being gote. If both edges of the territory are open-skirted, then playing on the left and the right may be miai, making the territory worth even less (see Do not defend territories open on two sides).
TODO: find better examples of more typical open skirt configurations.
White 1 may be a good move for other reasons, but as an attempt to make territory along the top it suffers from open-skirtedness. Black can jump into the open skirt with or a.
Charles gives a similar position of interest from a pro game. Black's territory at the top, if it can be called that, has an open skirt.
In this game Miyazawa Goro (B) - a player noted for his originality - played and later . Then the position was left for a while.