In order to get the notes "in between" the letters (where there is another possible fretted note to play) we use symbols in front of the notes we want to alter.

The symbol to play one fret higher (or musically one half-step higher) is called a "sharp" and looks like the number symbol "#". This is an accidental. The symbol to play one fret lower (or musically one half-step lower) is called a "flat" and looks like a lower case letter "b". This also an accidental. We put these symbols in directly in front of the notes we want to make sharp or flat.

After making a note sharp or flat, if we want to play it in its original unaltered position again later in the same measure (or piece) we use another symbol called a "natural" sign, which looks like a little slanted rectangle with some extended lines. This effectively cancels out the previous sharp or flat sign. This is an accidental as well.