Wolfram Language & System 10.3 (2015)|Legacy Documentation
The Wolfram System normally organizes the cells in a notebook automatically into hierarchical groups. Double-clicking lets you show only the heading cell of a group, or only a particular cell within a group—such as a graphic or the result of a computation.
Ctrl+G (Group Cells) — group selected cells together
Ctrl+U (Ungroup Cells) — ungroup selected cells