Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters ‘foobar’. Suppose you have read three characters so far. The situation looks like this:
f o o b a r ^
so the next input character will be ‘b’.
If instead of reading ‘b’ you unread the letter ‘o’, you get a situation like this:
f o o b a r | o-- ^
so that the next input characters will be ‘o’ and ‘b’.
If you unread ‘9’ instead of ‘o’, you get this situation:
f o o b a r | 9-- ^
so that the next input characters will be ‘9’ and ‘b’.