Notes can be written above & below the staff with the use of a ledger line.
A ledger line is like a temporary line used for a single note.
The higher or lower the note goes above or below the staff, the more ledger lines will be needed.
The note on one ledger line below the staff with a treble clef is middle C.
The note on one ledger line above the staff with a bass clef is also middle C.
Notes that require multiple ledger lines can be harder to read. This is the reason why alternative clefs such as the alto & tenor clefs are used for some instruments.
Notes on the lines and in the spaces do not account for every pitch available. There are other notes that fall in between these notes.
To show these notes we use one of the five different accidentals. Accidentals are always placed before the note and in the same space or line that the note uses.
If a key signature identifies certain notes to be sharpened or flattened in a key, a natural can be used in a bar to make the note unsharpened or unflattened.
A natural will only make the note a natural for the duration of the bar in which it is used or until a sharp or flat is placed with that note in that bar.
In the following bar, it then reverts to the original key signature.