Conventionally, guitarists double notes in a chord to increase its volume, an important technique for players without amplification; doubling notes and changing the order of notes also changes the timbre of chords. It can make a possible a "chord" which is composed of the all same note on different strings. Many chords can be played with the same notes in more than one place on the fretboard.
Unlike many string instruments which use a bow to create a sound, guitars are played by plucking or strumming the strings on the instrument, which vibrate to create a sound. Chords are played when the guitarist holds their fingers down on more than one individual string to change its length, which in turn changes the frequency of the vibration created - in other words, it creates a different note. Performers often use guitar tabs, rather than sheet music, when learning music: this acts as a kind of 'map' to teach guitarists where to put their fingers on the strings.