Do we define a walking bass by it consisting of mainly chord tones? So it's just a skeleton of a chord progression? What if it wasn't just chord tones, would it still be considered a walking bass line or just a regular bass line?
A typical walking bass is mostly composed of chord tones, but there will be some non-chord tones thrown in to add interest.
The most common non-chord tone will be the passing tone. Passing tones connect chord tones a third (sometimes fourth) apart by moving by step from one chord tone to the other. If the prevailing harmony is C7, for instance, the walking bass may play
C D E or
E F G, with the middle pitch in either case being a passing tone.
Another common trait of walking basses is the chromatic passing tone; this connects two chord tones that are a second apart, with each chord tone belonging to a different chord. Imagine, for instance, you move from a C7 chord to a Dm chord. Moving
C C♯ D really smooths out the motion and adds some chromatic interest. But be careful doing this; sometimes chromatic non-chord tones hurt more than they help!
Lastly, the bass player will usually want to emphasize the root of the chord. (They are, after all, the bass!) This root often appears on beat 1 of the measure, but not always. Otherwise, the remaining strong beats in the measure (like beat 3 in 4/4 time) will tend to have chord tones, while the weaker beats (beats 2 and 4 in 4/4 time) will tend to have the non-chord tones.
See also Walking bass line/chord help. It's a different question, but the answers there touch on your question, too.