A shop drawing is the detailed, plant-specific drawing of a precast piece. Where the structural drawings say what the finished structure has to do, the shop drawing says exactly how the plant will build the piece to make that happen: every dimension, the reinforcement, the embeds and lifting hardware, the blockouts, and the elevations the piece has to hit in the field.
The engineer of record reviews and approves the shop drawing, usually as part of a submittal, before any concrete is poured. That approval is the real gate in a precast schedule. Nothing gets cast to an unapproved drawing.
Because everything downstream describes the same piece, the approved shop drawing makes a natural single record for a job. Read the full guide, What a Precast Shop Drawing Is, and Why Approval Is the Bottleneck.