A ledger line is a short line used to write notes which would otherwise be too high or too low for the staff. A short line (slightly longer than the note) is drawn parallel to the lines on the staff, and the note head is placed on that line or in the space below or above it.
Notes more than two or three leger lines below the treble clef stave or two or three leger lines above the bass clef stage are rarely used. It is usually easier to change the clef.
Notes more than four or five leger lines above the treble clef stave or below the bass clef stage also become hard to read. These very high or very low notes are easier to read if an “8va” sign (called "ottava" sign) followed by little dots is placed above or below the notes. This means they are to be played an octave higher (or lower) than written.
The piccolo always sounds an octave higher than the music written and the double bass always sounds an octave lower than written. Some wind instruments also transpose up or down an octave to make it easier to read, e.g. the bass clarinet sounds a ninth (just over an octave) lower than the music which is written in the treble clef (this makes it the same fingering as the clarinet).