piping (Cambridge Dictionary)
- a folded strip of cloth, often enclosing a cord, used to decorate the edges of clothes or furniture
In the context of "decorative clothing", the idea is that the strip of cloth is folded into what looks like a thin pipe or tube. Note that in a culinary context, you might decorate a cake with sugar and/or cream-based "piping" using a "piping bag" (a fabric bag containing material forced out through a pipe, a bit like a grease gun).
Tassels are tufts of loosely hanging (decorative) threads or cords, whereas piping is securely attached / stitched throughout its length on to the main bulk of the thing being decorated (clothes, chair covers, curtains, etc.).