If you pluck any string of the guitar, it will produce a tone - a single musical note. A single note can vary in pitch (how high or low the note is). On the guitar the pitch is determined by where the string is fretted.
Fretting a string near the nut (towards the headstock) will produce a low-pitched note. Fretting the string higher up the neck (towards the body) will produce a higher-pitched note.
Pitch is also determined by which string we are plucking. The thickest string (6th string), when played open, produces the lowest possible note on the guitar, while the thinnest string (1st string) produces a higher pitched note.
Notes can also be sharp or flat: If we take the note A and raise its pitch by one semitone, it will become A# (A sharp). On the guitar this means raising the note by one fret (one semitone = one fret). If we lower the note A by one semitone (one fret), it will become Ab (A flat).
This gives us all the notes we need to play and understand music, twelve notes in all.