When you start terminal, your in your home directory, usually
/Users/<some username>
You can type in
pwd
Which shows you your Present Working Directory (where you are on the disk)
Then type in
cd /
which brings you to the root directory.
When you do an ls there, you will see those directories.
ls -a shows you any files that begin with a "." (period)
Those are more or less hidden.
ls -al shows all files in long format which gives you permissions, owners and file sizes.
don't go touching things though...