These are tools to create parent-child relationship in the work break down structure defined in a project.
The meaning of Indent is – Making a child task as a parent task.
Out-dent is – Making a Parent task as a child task.
The Outdent and Indent buttons are available on the left-hand side of the Formatting toolbar and are used to create an outline in a project plan. The outline buttons allow the outline symbols to be toggled on and off. Options for displaying outline numbers, symbols, name indentation, and summary tasks are available in the View tab (from the Tools menu select Options). The equivalent outlining commands are also available from the Outline submenu on the Project menu.