The unaccented passing note literally passes between two notes a third apart (in other words, it fills in the gap) . An important characteristic of the passing note in the example below is that it is dissonant. While the b1 and g1 in the soprano voice are both consonant withe chord I (G), the dissonant a1 'passes' in between.
As explained in prescribed progressions, dissonant notes are usually subject to various voice-leading 'rules' depending on the style. An unaccented passing note occurs by definition on an unaccented part of the bar, so the dissonance is not very prominent.
Because it fills in a third, the passing note below can only be approached and left by step. But the 'rule' that dissonances must be resolved in descending motion is relaxed - can be found ascending or descending. The stepwise motion and lack of accent allow this rule to be broken without the dissonance becoming obtrusive. The unaccented passing note is therefore the most common embellishment in styles where too much dissonance is undesirable.
The first two examples below show the same chord progression with and without a passing note while the third shows the progression reversed so that the passing note ascends.
The two notes that passing note passes between need not be part of the same chord as in the above example (V and V6). Intervals larger than a third may also be filled with a number of passing notes. You will find many examples in the main body of the site.