The work breakdown structure (WBS) is the order and hierarchy of the tasks in your Gantt view: which tasks sit under which summary tasks, and in what order. Locking the WBS stops tasks from being moved, indented or reordered by accident.
Lock the WBS
Go to Actions > Lock WBS, or press Ctrl+Shift+L.
Unlock and reorder
Go to Actions > Unlock & Reorder WBS, or press Ctrl+Shift+U.
Move tasks with Move Up / Move Down (Ctrl+↑ / Ctrl+↓), and change the hierarchy with Indent / Unindent (Ctrl+→ / Ctrl+←).
Lock the WBS again when you're done.
Locked tasks
A lock icon in the first column of the task grid means that task is locked, so its dates and details can't change by mistake.
To unlock a single task, open it (double-click or F2) and click the lock icon at the bottom of the Task Properties panel. See How to Unlock Tasks and Tags.
Tip
Use Actions > Expand All and Collapse All to see or hide every level while you reorder.