This command is used to change the current working directory. Try changing to your a01 subdirectory with
cd a01Then use pwd to verify that you have changed the current working directory. If you just created the directory, you should have no files there, so typing lsshould produce nothing and ls -a should give only ./ and ../.
To go back to the parent directory, do
cd ..and then use pwd and ls to verify the change. At this point, you might like to use cd .. to back up still one more step above your home directory. This operation will place you in the directory containing all home directories for this class. If you now do ls you will see a listing of all usernames for this class.
To return to your home directory, do
cd ~with no argument. Use pwd to verify the result.