What is bundling?  "Bundles" are groups of standards arranged together to create the endpoints for units of instruction. Bundling is just one step in a curriculum development process; many other steps are required to create instructional materials designed for the NGSS.

Why bundle?  Bundling is helpful step in implementing standards because it helps students see connections between concepts and can allow more efficient use of instructional time. 

As is highlighted in the Introduction and Guide, there are a variety of things to consider in thinking through how to build bundles for courses.  

These example bundles are not intended to represent the only ways to organize courses, but rather to provide some well thought out, concrete examples of what bundles might look like.

For each grade, two different examples are provided to motivate users to compare the different examples and think about what works in their local context:

• For each grade K-5, an example is provided that organizes the bundles based on the Topics arrangement of the NGSS and one that focuses on a particular theme that builds across each year.
• For grades 6-8, an example is provided that bundles based on the Topics arrangement of the NGSS, and one that bundles by focusing on phenomena that hold standards together. 
• The examples for grades 9-12 are built from two of the Model Course Maps of Appendix K.

(Information copied from nextgenscience.org)