A note (ha!) about chords: A chord is, in the simplest definition, two or more notes being played at the same time. And it doesn't have to be the same musician playing those notes. If I play a C, someone else plays a G, and a third person plays an E, the listener hears the sound of a C Major chord.
As bassists, we are usually charged with playing the lowest note of the chord. But if we only played the roots of chords, our bass lines would be utterly lifeless. So we can spice up our "roots" with chord tones and scale tones (remember, every scale contains a chord, and every chord represents a scale). Country players do this with the stereotypical 1-5-1-5 bass line (What road did the country bass player live on? ROUTE 5!), blues bassists use the also-stereotypical 1-3-5-6-b7-6-5-3 bass line. So even though we as bassists rarely play more than one note at a time, we absolutely MUST know the notes in a chord or scale in order to make our bass lines have meaning and interest... and we have to know what chords everyone else is playing at a given moment, because that informs our note choices. There's a reason a B sounds terrible over a C7 chord; conversely, there are ways to utilize that same B if you're careful and judicious.
Chord tones are essentially the 1, 3, and 5 of a given scale at its most basic level. They get more complicated as more notes are introduced to the chord, but there are essentially 4 "triads" and 2 other chords to master before you get deeper in:
- Major - 1 3 5
- Minor - 1 b3 5
- Augmented - 1 3 #5
- Diminished - 1 b3 b5
There is also a thing called a "suspended" chord, where the notes are either "1 4 5" or "1 2 5" (sus4 or sus2, respectively).
You absolutely must be aware of what chord is being played at any given time in the song, and respond accordingly, or you'll make your bandmates sound bad. There is no grey area here; ya gotta do it.