What you see above is the total number of notes possible using a five-line staff. In this case, we see an example that uses treble clef, making the lowest note D, just below the staff, and the highest note G, just above the staff. There are a total of eleven notes possible. The problem is that virtually every musician is capable of producing (with an instrument or her voice) far more than eleven notes. So what do we do if we need to notate a pitch that exists above or below the staff? Enter the ledger line (or leger line, depending on who you are talking to).
When we need a note above or below the staff, we place a small line in order to extend the staff temporarily. An example of a ledger line is shown above in red.
Now we can place the A note onto the ledger line to continue above the G. If we need higher notes, we simply continue to add ledger lines when necessary.
Similarly, we can place a ledger below the staff to notate a pitch that is lower than the staff, as you can see here with the note C one ledger below the staff.
These are examples of notes placed on ledger lines above and below the treble clef.