A passing tone is a note or notes placed in between two notes of a triad. So if you were playing over a G major chord and you played this.


e---------------
b--12-14-15-
g---------------
d---------------
a---------------
e---------------


'14' is the passing tone between the notes B and D of the G major chord. '14' is also the b5 over a G major chord. In the above example, this note is a passing tone and is commonly found in rock/country soloing. In Blues, '14' might be the only note played when G major is played in the harmony. Then it is often bent up a semi-tone (to D) to make it consonant.

There are several versions of passing tones. The quote Passing tone unquote is the linear version as seen above. There are other where you go up and then down to the note of a triad. It would be good to look into them, it will give you some ideas.