A certificate of incorporation is the founding legal document that creates a corporation, filed with the state, establishing the company's name, its authorized shares, and its basic corporate structure.
For a Delaware C-corp, the most common startup structure, this document sets the total number of authorized shares (which has to be enough to cover founders, an option pool, and future investors without needing an amendment every round), and it establishes the classes of stock the company is allowed to issue, including any preferred stock created later for investors.
Amending it later, to authorize more shares, create a new class of preferred stock for a funding round, or change other foundational terms, requires a shareholder vote and a new filing with the state. Worth getting the initial structure reasonably future-proofed rather than needing amendments for routine, foreseeable events.
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