cal shows the calendar.  It shows the dates, years and months.

free shows memory. It shows how much memory is available.

cd /usr changes directory.

pwd shows home. pwd shows the current directory you are working on.

ls -a shows all files.  ls -a shows all files on the computer.

file shows directory. file shows all files under the current user.

less shows to read a file

df shows how much disk space is available.