Frequency is the most important characteristic of sound. It determines the "highness" or "lowness" of the sound. Because of the difference in frequencies, we can write melodies, chords, and entire compositions. Instead of numbers, in musical notation, the frequency is defined by the placement of musical notes on the staff.
Accidental symbols can modify frequency. They affect all notes with the same name in a measure. We usually only use three types of accidentals, but there are five of them. Sharp and flat respectively raise and lower a note by one semitone. They can also be double. The natural symbol cancels other accidentals.