Lazy streams and infinite series
No, that's not what the author means. The author means the specific sequence of characters the user will actually type. If the user is about to write a letter to Al that might be:

'D'::'e'::'a'::'r'::' '::'A'::'l'::rest


The point is that you conceptually have "all" of the characters, and you can (with a bit of care) process this list like any other list.