CSS Flex Direction

  Flex direction controls **flow of items**.
  
  ## Direction values
  - row (default)
  - column
  
  ## Example
  ```css
  .container {
    display: flex;
    flex-direction: column;
  }
  ```
  
  ## UI Visualization
  - Items stack vertically
  - Like a column layout
  
  ## Real-world example
  - Sidebar menus
  - Mobile layouts
  
  ## Key understanding
  Direction changes main axis.