Shell chords are a “simpler” way to play 7th chords (all families, maj7, dom7, min7 etc…). To obtain a shell chord we cut the “fat” by omitting the 5th degree of the chord. So you retain only the root, third and seventh degree of the chord.
Why are we omitting the 5th degree? By doing so we make the sound of the chord less bottom heavy AND you economize on fingers (you only have 5 fingers from which you use only 4 most of the time ). Fingers that you can use for more interesting notes, like a 9th, 11th or 13th later on. (video bottom of the page)