do man -du. Unfortunately, the description's format does not make this
clear.

Second, the | symbol means ``or.'' So you can use the -c, the -w, or the
-tZT option, followed by a device argument.

Third, notice that you can nest the brackets, because they indicate
optional units. So if you have a section, you must also have a page,
because e page is not optional within the [[section] page] unit.

There's no need to memorize any of this, just refer to this section as you
read documentation.

Files and Directories

Files are a facility for storing and organizing information, analogous to
paper documents. They're organized into directories, which are called
folders on some other systems. Let's look at the organization of files on
a Debian system:

/.
A simple / represents the root directory. All other files and
directories are contained in the root directory. If you are coming
from the DOS/Windows world, / is very similar to what C:is for
DOS, that is the root of the filesystem. A notable difference
between DOS and Linux however, is that DOS keeps several
filesystems: C: (first hard disk), A: (first floppy disk), and D:
(either CD-ROM or second hard disk), whereas Linux has all its
files organized above the same / root.