ShowAutoStyles is an option for Cell which specifies whether styles that are specified to be automatically used for various syntactic and other constructs should be shown.
The default setting is ShowAutoStyles -> True.
Details of automatic styles can be specified in the setting for AutoStyleOptions.
For example, unmatched delimiters such as brackets are by default shown in purple.
See The Mathematica Book: Section 2.10.11.
See also: StyleBox, DelimiterFlashTime, ShowCursorTracker.