Chords are the tools that musicians use to accompany melodies for improvisation - they form the harmonic underpinning of all music.
A chord is any collection of 2 or more notes that are sounded together. The 1st note of any chord (i.e. the note that gives the chord its name) is called the root note. The most common of all chords types are 3 note chords knowns as triads.