IgnoreCase is an option for string manipulation and searching functions which specifies whether lower- and upper-case letters should be treated as equivalent.
With the default setting IgnoreCase -> False, lower- and upper-case letters are treated as totally different.
With the setting IgnoreCase -> True, lower- and upper-case letters are treated as equivalent.
IgnoreCase is an option for StringPosition, StringReplace, StringMatchQ, Find and FindList.
IgnoreCase in no way affects the parsing of Mathematica expressions.
See The Mathematica Book: Section 2.7.2.
See also: ToUpperCase, ToLowerCase, SpellingCorrection.