Org is implemented on top of Outline mode. Outlines allow a document to be organized in a hierarchical structure. An overview of this structure is achieved by folding (hiding) large parts of the document to show only the general document structure and the parts currently being worked on.
<TAB>(org-cycle &optional ARG)
This function has many different semantic meanings depending upon its context and how many arguments precede it.
When point is at the beginning of a headline, rotate the subtree through 3 different states (called local cycling): FOLDED -> CHILDREN -> SUBTREE
When called with a single prefix C-u, rotate the buffer through 3 states (global
cycling): OVERVIEW -> CONTENTS -> SHOW ALL
When called with two prefix arguments (C-u C-u) switch to the startup visibility.
When called with three prefix arguments (C-u C-u C-u) show the entire buffer, including
drawers.
<S-TAB>(org-shifttab & optional ARG)
Global visibility cycling (unless within a table; then (org-table-previous-field);