Answer:
A sharp sign means "the note that is one half step higher than the natural note". A flat sign means "the note that is one half step lower than the natural note". Some of the natural notes are only one half step apart, but most of them are a whole step apart. When they are a whole step apart, the note in between them can only be named using a flat or a sharp.
In music, flat (Italian bemolle for "soft B") means "lower in pitch". Flat is the opposite of sharp, which is a raising of pitch. In musical notation, flat means "lower in pitch by one semitone (half step)", notated using the symbol ♭ which is derived from a stylised lowercase 'b'.
In music theory, a natural is an accidental which cancels previous accidentals and represents the unaltered pitch of a note. A note is natural when it is neither flat (♭) nor sharp (♯) (nor double-flat nor double-sharp.
