How were merchant ships protected in World War 1?

Your teacher is probably referring to the "Convoy System" that was developed by the British Royal Navy & used by the Allies. It meant that merchant ships with the same cruising speeds traveled together in a group or "convoy". The group may be escorted by Navy warships that were capable of protecting the merchants from German U-boats, Q-ships, or surface raiders. Ships that were very slow, or fast would not travel in a convoy.