The term "open string" means a string with no left hand fingers pressed down (unfretted). When correctly tuned, the open strings of the guitar correspond to the notes E B G D A and E from high to low. These notes are shown in the following fretboard diagram.
Notice that the open 4th string D note is in the space below the staff, while the low A and E notes are on ledger lines below the staff. Don’t worry if you can’t recognise these notes yet, they will all be introduced as the book progresses. An easy way to remember the names of the open strings (from high to low) is to say Easter Bunny Gets Dinner At Easter
Here is an exercise to help you recognise the notes which represent the open strings of the guitar. Name the notes out loud as you play each one. All the notes here are quarter notes, which last for one beat each. However, don’t worry about the timing at this stage, just make sure you are playing the correct notes.