This tells MySQL to get the current date and time and display it. You'll see something like this appear:

+--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | test | +--------------------+ 4 rows in set (0.00 sec)

This command lists all the MySQL databases on your computer. Since you've just installed MySQL, there will just be a few default databases, similar to the following:

+---------------------+ | now() | +---------------------+ | 2011-08-24 11:36:40 | +---------------------+ 1 row in set (0.00 sec)

Now try another command:
show databases;