[Haskell-beginners] questions about "Write yourself a Scheme in 48 hours"

[Michael Litchard]

I'm working through "Write yourself a Scheme in 48 hours". I'm on the
parsing section
http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours/Parsing

and I have a question about the following function

parseNumber :: Parser LispVal
parseNumber = liftM (Number . read) $ many1 digit

digit looks like a variable, but I don't see it mentioned anywhere.
Where is the input coming from?
Is this an example of partial application?