The lead investor anchors a financing. They negotiate the term sheet, set the price, take the largest single check (often 50%+ of the round), and usually take a board seat or observer role. Other investors in the round (the syndicate) accept the lead's terms.
Choosing the right lead is one of the highest-leverage decisions a founder makes. The lead becomes the most influential outside voice on the company for years. Founders should evaluate leads on partner fit, value-add beyond capital, and prior portfolio support, not just valuation.