What part does negotiation play in supply chain management?

It's actually quite simple.  If you work in purchasing or procurement, a key part of your role will be to track inventory levels and order new components, equipment, or raw materials as needed so your employer can fulfill orders coming in from clients.

This means identifying the best suppliers for the products you need and negotiating the best deal you can in terms of price, quality, service, conditions, etc.  

In most cases, you'll talk to several different suppliers and compare deals before deciding on who your company will do business with.