A passing chord is a non diatonic chord that connects, or passes between, the notes of two diatonic chords. In the case of your sub dominants, they won't affect the overall tonality of the diatonic progression because they function as little cadences.
If you applied longer durations to each chord you'd find that those 'passing chords' begin to take on more weight and are no longer a part of the original tonal center.
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