If you enter <b>n</b>, for example, the user will only be able to enter <i>one</i> number. This limits the amount of information they can enter. So as an example, if you wanted a (5 character) 'Zip Code' field, you would put the expected input format as: <b>nnnnn</b><br/>
Anything besides 5 numbers would be rejected. They would not be able to enter more or less numbers either.