Almost all music is centered around chords. Chords define the harmonic structure of each song and tell you which notes will sound good and which won't. If you study music theory, you'll spend a lot of time learning about what the different chords are and how they lead from one to another.
Guitarists and pianists play full chords, simultaneously sounding every note that makes up each chord. They are the ones who really fill out the harmonies. As a bass player, your relationship with chords is a little different. You don't play every note in a chord, but your deep, low tones ground the chord and help define its sound.
What are Chords?
A chord, by definition, is a group of two or more notes played together. Generally, it is three or four notes and they are separated from each other by intervals of major and minor thirds. Each chord has a root note, the foundation upon which the chord is built, and a "quality," the structure of the other notes that make up the chord. For example, a C minor chord has the notes C, Eb and G. Its root note is C and its quality is "minor."
There are many qualities of chords. Some examples are major, minor, major seven, minor seven, diminished and augmented, and the list goes on. Each one has a different character, created by the different musical intervals between the chord tones (notes in the chord).