CHOWN: What does "id -u" represent

I am working to get Mongodb running on a Ubuntu server install. In reviewing the instructions I needed to create a "\data\db" directory in the root drive. At which point I needed to alter the owner using the CHOWN command as follows:

sudo chown `id -u` /data/db

When I issue that command as it appears in the quick start guide I receive

chown: invalid user: 'id -u'

I am new to Linux, so what I don't understand is what the 'id -u' was supposed to mean. When I replace with my user name the command completes just fine and mongo runs. Can someone help me understand what the short hand 'id -u' would communicate to an expert Linux user that it did not to me?

The command id -u prints out your "numeric user ID" (short: UID); as you already noticed, it is the same as spelling out your username in full on the chown command line. Indeed, the following command invocations should all have the same effect:

sudo chown `id -u` /data/db
sudo chown $USER /data/db

The reason why it did not work as expected has likely to do with the quotes: they have to be backquotes (ASCII char 0x60), whereas the chown error message suggests that you used single quotes (ASCII char 0x27).

You can find a very thorough explanation of UNIX shell quoting here.