Like Stuart has said, it's a spanner to tighten the leg clamps if/when they work loose. For most of the monopod's life, it's not really important but for that one time when the leg starts slipping slowly through the clamp, it's VITAL! Either tie it to the monopod of shove it somewhere in your kit bag - you never know when you'll need it, apart from that it'll be the day after you've lost it or when it's at home...