A shell voicing uses three notes: root, 3rd, and 7th. These are the guide tones — the 3rd defines major vs minor, the 7th defines the chord extension. Together they tell you everything about the chord's quality while leaving maximum space for the bass player and soloist.
Root (1P) + 3rd (3M or 3m) + 7th (7M, 7m, or 6M for sixth chords). Three notes total, left hand.
Your default comping voicing in any small group setting. Essential for beginners — learn these before anything else. Use when sight-reading, when the texture needs to be light, or when playing solo piano where you need the root audible. Every jazz pianist starts here.