ASC and DESC stand for ascending and descending and refer to the second drop down box - so Price, Date Listed, and Creatu Type.
Ascending means they will be sorted from bottom to top (e.g. Price will go from lowest to highest) and descending means they will be sorted from top to bottom.
I believe the default is Date Listed in ascending order, so it'll show the oldest listings through to the most recent.
Hope this helps! If you need any more help feel free to ask :)