Smart Highlight selects all occurrences of the current word (variable) in surrounding text. To activate it you can use the main menu item, shortcut (can be defined) or Ctrl + left mouse Click on word. In addition to simple search it has some smart logic:
Additional Smart Highlighting styles makes it possible to have multi style permanent smart highlighting displayed in code in same time. This is added in addition to automatic, not permanent smart highlight.
You can enable the feature in Options->Code Completion.