This series of lessons on guitar chord inversions begins with triad inversions.
Chord inversion is simply where the stack of chord tones is rearranged so the root (1) is no longer in the bass position. This means one of the other chord tones (3 or 5 in a major triad) occupy the bass.
This restacking of the chord's notes gives the chord a different sound (voicing) and allows you to play that same chord in several positions across the fretboard.
By mastering inversions, you not only widen your chord vocabulary, but you also have the option of using more economical fingering through your chord changes, as certain inversion shapes will exist higher/lower on the fretboard, closer to other chords in the progression.