A cap table tracks who owns what in a company. At minimum, it lists each shareholder, the share class they hold, the share count, and the percentage ownership on a basic and fully diluted basis. Modeled cap tables also include round-by-round projections, option grants, and conversion mechanics for SAFEs and notes.
Clean cap tables are a prerequisite for institutional rounds. Common problems include unrecorded option grants, lost paperwork on early SAFEs, and informal verbal commitments to advisors that haven't been papered.