ima ... I don't use one but it is a basket that catches the loose line as you strip it in . Or when you strip slack line out prior to casting . I keeps the line neat and prevents tangles so that when you cast you dont have to worry about the line getting caught on your gear or floating downstream and getting snagged . It folds into a neat pile in the basket . I havent described it very well but you almost have to see the construction of one to get the idea of how it prevents tangles . In a boat it keeps you from putting a loop around your feet , the oarlock , some gear , or an oar etc . which can result in you tossing you gear overboard when you cast.