Trim wraps are a decorative thread pattern typically used around the edges of your guide wraps.
Ideally, you would use a contrasting nylon thread or metallic thread for this. For example, if you are using black guide wrap, then you may choose a red or gold thread for your trim wrap.
Also, trim wrap thread is typically size A thread. Size A gives the trim wrap a tight finish that almost looks like a solid color instead of threading (like if you used a larger size thread).