Definition: 

The “Iron Triangle” is meant to show interdependent relationship between scope, cost and schedule. This triangle models constraints of project management. These are considered as “iron” because your can’t change one constraint without impacting the others. This was originally proposed by Dr. Martin Barnes in 1969. Scope is fixed and resources and time are variable, this means team starts with fixed requirements which determines the project scope. The resources and schedule are variable, estimated depending on the fixed scope.

Scope is the work to be done, such as features and functionalities, resource include budget and team members working to deliver and execute, time is when teams will deliver to market such as milestones or release dates. When teams are asked to deliver fixed scope, and if they are not in a position to deliver then the only variables they can play with are time and resource which impacts the cost. As transitioning to agile ways of working, key project stakeholders need to learn to make changing scope their first choice, easier to lock down the schedule and resources without compromising in quality. 

Further Reading:
Book: SUCCEEDING WITH AGILE Software Development Using Scrum by Mike Cohn