The 'Select' method is what in most other languages is known as 'map'. It enforces a projection on each item in a sequence. Since 'string' is a sequence of 'char' elements, it can be used to project each 'char' element. The projection used here is a mapping from 'char' to 'int' (Dictionary<char, int>). An indexer '[x]' is used to project each 'char' of our 'string' as key to this mapping to get the number corresponding to the roman glyph.