A chord is a group of notes.
C chord is made up of the notes:
C E G
Take note: The above image is NOT a standard chord diagram.
Using a typical open chord of C, the notes will be: E C G E C x
C is the bass note, i.e. the lowest note in the chord. If you play the low E note, i.e. 6th string, the chord name will be changed to C/E, because E is now the lowest note.
However, if we ignore the naming rules, as long as there are the three note of C, E and G, it is considered a C major chord! It is still a C chord even if you jumble up the sequence of the notes.
There are various systems of understanding chords, and they deserve a better explanation in another article. As for now, the take home point is: a chord is a group of notes.