It all depends on what data is being sent to serial2, and what you are trying to accomplish with it.
If you have a look at the ascii table that mmiscool has helpfully included at the bottom of the online docs, you'll see that all the numbers from 0 to 31 are translated as control codes (which do not display), eg: number 13 causes a carraige return when 'printed' to screen.
You'll notice that ascii 0 (zero) is interpreted as NULL, whereas the printable ascii character "0" is represented by the decimal number 48.
So you can see that if you want to display incoming numeric digits (eg: for dev and debugging purposes) you can add 48 to each serial digit to turn it into it's printable ascii equivalent - but bear in mind that non-separated consecutive digits (such as 32) would originally comprise a multiple digit decimal (or hex) number.