To play rythym guitar well, it is obviously important to be able to change from one chord to the next smoothly and accurately. Perhaps the biggest hurdle to this is simply "connecting the brain to all of the fingers." After all, simultaneously moving all of your fingers to the next notes of the chord can be overwhelming.
When moving from one chord to the next, especially newly learned chords, most students will lift all or most of their fingers off of the fretboard entirely. In a way, it is sort of a 'get a fresh start on the next chord' mentality. Unfortunately, this creates more work and confusion for both your brain and your fingers.
To help cut down on the extra brain work and overall hysteria that comes with changing chords, we use the notion of pivot notes, sometimes called pivot tones. Put simply, these are notes that are played with the same finger in the exact same spot between two successive chords in a progression. Any fingers that are playing pivot notes will not move with the rest of the fingers. A given pair of chords in a chord progression may have one pivot note, mutiple pivot notes, or none at all.