As the prior poster indicated /dev/null means "nowhere" essentially. This is called the "bit bucket". Redirecting to /dev/null is just a way to prevent output from showing up anywhere else - you're simply discarding it by sending it to the bit bucket.
I was saying that UNLESS you do a redirect the default for most stdout and stderr is the screen. Almost all redirects to /dev/null are simply to prevent output from appearing on the screen (or in log files when run in the background).
Redirects can be done TO or FROM files. In UNIX/Linux nearly everything is called a "file" including "device files" such as /dev/null, /dev/sdb, /dev/st0. You could redirect into a "regular file" called /tmp/mkfs.log if you wanted by modifying your original line as:
mkfs -t ext3 /dev/sdb1 < fd.data > /tmp/mkfs.log 2> /tmp/mkfs.log
mkfs -t ext3 /dev/sdb1 < fd.data > /tmp/mkfs.log 2>&1
After you ran the command you could type "cat /tmp/mkfs.log" to see what kind of output it had.
I don't know off the top of my head what the entries in your fd.data file are doing. It's possible it isn't doing anything because it doesn't understand what was sent to it. Running to a log might give you better insight. (Alternatively you could simple NOT do the redirects to stdout and stderr and it would go to the screen.)
However, since you don't know what /dev/null is it appears you really need to drop back and NOT muck with this. mkfs will overwrite your /dev/sdb1 partition if you have anything on it already.
You might want to do a search for "Linux tutorial" - there are many places on line to read for example this one:
It is OK to ask questions but you really ought to work your way up from basics to more advanced topics and ask questions as you go along. You don't need to know everything all at once but if you don't know about /dev/null or redirects you DO need to go and learn about the basics before you do some damage to your system.