When we play more than one note simultaneously, we are creating a harmony.
On the guitar, we can play anything from two to a maximum of six notes simultaneously (a different note on each string).
If we play two notes of the same pitch but an octave apart, we create a stronger sound than just by playing a single note alone. This is because the waveforms and harmonics created by striking the two notes will spread over a wider frequency range than if we just played one note.
If we play two notes of different pitch, we will hear a combination of both notes in one sound. This can sound pleasant or it can sound dissonant, depending on what notes are played, and the intervals between them.
This is the basis of harmony in music, seeing how a single musical note sounds against another, or against several others.
As we keep adding notes, the dynamics of the chord will become more and more complex. If we play a seventh chord, for example, we will be hearing a combination of four individual notes simultaneously. The more notes that are in a chord, the more "colour" it will have.
If we play a major chord, the sound produced will suggest a 'major' sound, which can be defined as being strong, happy, lively.
If we play a minor chord, we hear a 'minor' sound, which can be defined as being sad, melancholy, spirited.
The differences in emotion we hear from these two chords are created because of the harmonies of the notes involved.