What is triple constraint triangle in project management?

Project cost, schedule and scope are depicted as a project management triangle. The triangle is formed by customer as the center point and the three aspects form the sides of triangle. In order to gain customer satisfaction, the project management team should deliver scope in propose schedule and cost. In case any leg is gets disturbed, the other two legs gets affected. For example, if the customer increases the scope, then the other two legs-schedule and cost- get affected a lot.