Understanding the relationship between the core components in the STEM cycle is very helpful to understanding—and applying—the three dimensions: practices, disciplinary core ideas, and crosscutting concepts that we find within the Next Generation Science Standards.
The NGSS present a clear directive for what students should get from STEM education: Effective STEM teaching will result in learning where a student can demonstrate an understanding of this standard in any relevant context.
The crux is this: A student can demonstrate what they have learned in any related context. In this way, the NGSS performance expectation is like the seat of a stool with three legs (the three dimensions) holding it up. Those legs not only support the standard, they help form the context in which the students will be expected to demonstrate understanding.
The three "legs" are the three dimensions:
Science and engineering practices: This is a skills dimension, something you may be familiar within the Common Core math practices. The practices demonstrate skills specific to the discipline—in this case, STEM— and help form processes, becoming a means of accessing background content.
Disciplinary core ideas are the content leg. NGSS content was chosen because it’s dynamic and interacts other areas of content in a system. Many parts interrelate and pull different areas into contact such as life science and earth science. Disciplinary core ideas are intended to scaffold this way.
Crosscutting concepts are akin to systems thinking. They relate to how the content behaves in a system. Consider an ecosystem as a metaphor. You can think about the way matter moves in nature, between plants, animals and decomposers, and understand it as a whole or in parts. These parts interrelate as elements of the water cycle, life cycle, food chains, and food webs.