Skeleton animation sprites can have "slots" where you can place "attachments". An attachment is simply an image that can used as a prop by the sprite, and is placed in a slot so that it will move along with the animation of the part where the slot is positioned. If you imagine the sprite of a warrior with an attachment slot on his hand - you can then place different weapon sprites on this slot and make the warrior look like he's using a sword or a pike or a shield or whatever.
This section deals specifically with attachments and their sprites, however you can also setup slots and get information about them from the functions listed on the Slots page.
The available attachment functions are: