C#/ .NET - what is white character?

White characters are characters that are not visible like letters, numbers, symbols or emoji.

So, it can be: space, new line, tabulator, etc.

From Microsoft Documentation:

White space characters are the following Unicode characters:

• Members of the UnicodeCategory.SpaceSeparator category, which includes the characters SPACE (U+0020), NO-BREAK SPACE (U+00A0), OGHAM SPACE MARK (U+1680), EN QUAD (U+2000), EM QUAD (U+2001), EN SPACE (U+2002), EM SPACE (U+2003), THREE-PER-EM SPACE (U+2004), FOUR-PER-EM SPACE (U+2005), SIX-PER-EM SPACE (U+2006), FIGURE SPACE (U+2007), PUNCTUATION SPACE (U+2008), THIN SPACE (U+2009), HAIR SPACE (U+200A), NARROW NO-BREAK SPACE (U+202F), MEDIUM MATHEMATICAL SPACE (U+205F), and IDEOGRAPHIC SPACE (U+3000).

• Members of the UnicodeCategory.LineSeparator category, which consists solely of the LINE SEPARATOR character (U+2028).

• Members of the UnicodeCategory.ParagraphSeparator category, which consists solely of the PARAGRAPH SEPARATOR character (U+2029).

• The characters CHARACTER TABULATION (U+0009), LINE FEED (U+000A), LINE TABULATION (U+000B), FORM FEED (U+000C), CARRIAGE RETURN (U+000D), and NEXT LINE (U+0085).

References

1. Char.IsWhiteSpace Method: Remarks - Microsoft Docs