Sheila is explaining the importance of placing the pins. Lacers tension the threads against the pins and it is that tension and those pins that determine how the finished piece of lace will look and how high the quality of the finished work. Bobbin lace has no knots, so poor tension or erratic pin placement can leave the lacer having done a lot of work, but resulting in only a sloppy piece to show after all that time and effort. Learning a feel for tension and carefully ensuring precise pin placement are key elements of learning how to produce quality bobbin lace.