A board of directors is the group with formal legal authority over a company's major decisions, hiring and firing the CEO, approving fundraising, approving a sale, elected by shareholders rather than appointed by the founders alone.
A board seat is a specific voting position on that board, and it's one of the most consequential things investors negotiate for in a term sheet. An investor holding a board seat has a direct, ongoing say in the company's biggest decisions, not just a passive financial stake.
Founders sometimes underweight how much board composition matters compared to valuation, since it doesn't show up as a number on the cap table. A board that's lost founder control can make decisions, including who runs the company, that the founders themselves disagree with, which is exactly why board seats are worth negotiating as carefully as the price.
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