Skirt is a plugin to give the extruder some extra time to begin extruding properly before beginning the object, and to put a baffle around the model in order to keep the extrusion warm.
It is loosely based on Lenbook's outline plugin: http://www.thingiverse.com/thing:4918
It is also loosely based on the outline that Nophead sometimes uses: http://hydraraptor.blogspot.com/2010/01/hot-metal-and-serendipity.html
And also loosely based on the baffles that Nophead made to keep corners warm: http://hydraraptor.blogspot.com/2010/09/some-corners-like-it-hot.html
If you want only an outline, set Layers To to one. This gives the extruder some extra time to begin extruding properly before beginning your object, and gives you an early verification of where your object will be extruded.
If you also want an insulating skirt around the entire object, set Layers To to a huge number, like 912345678. This will additionally make an insulating baffle around the object; to prevent moving air from cooling the object, which increases warping, especially in corners.
The default 'Activate Skirt' checkbox is off. When it is on, the functions described below will work, when it is off, nothing will be done.
When selected, the skirt will be convex, going around the model with only convex angles. If convex is not selected, the skirt will hug the model, going into every nook and cranny.
Gap over Perimeter Width
Defines the ratio of the gap between the object and the skirt over the edge width. If the ratio is too low, the skirt will connect to the object, if the ratio is too high, the skirt willl not provide much insulation for the object.
Protip: If you're printing an object that has little surface contact with the bed and are having issues with it coming loose, you can set Gap over Perimeter Width low so the skirt touches the object, which will help anchor it to the bed. When you're done printing, just trim the skirt off.