
When the OTTAVA SIGN is placed over notes, they sound an octave higher. When the OTTAVA SIGN is placed below notes, they sound an octave lower. This extends the range of the staff without using a lot of ledger lines.
POINT (Don't click) YOUR MOUSE at the following notated pitches. The sounding pitch is actually an octave higher (g2) on the first example but lower (b) on the second.
and 
The OTTAVA SIGN can be used above or below groups of notes, too. POINT (Don't click) YOUR MOUSE at each group of notes in the example below. As you can see, the OTTAVA SIGN allows us to put notes on the staff and still expand the range of the staff without using ledger lines. When you see notes with an OTTAVA SIGN over them, you are actually supposed to perform them one octave higher than written. Similarly, when you see notes with an OTTAVA SIGN below them, you are to perform them one octave lower than written.

You may find it helpful to draw some of the symbols that have been presented thus far.