Capital gains or losses are generally from the sale of stocks, bonds, property, or other "capital assets". If you have a capital loss that you couldn't fully claim on your Arkansas return in a prior year and can carry it forward to this year, enter it here.
If you don't have one, don't enter a zero, leave the box blank. Entering zeroes when you aren't specifically instructed to enter zero can cause errors that prevent you from e-filing.