A lead investor is the firm that anchors a financing round by setting the valuation, writing the largest check, and taking a board seat. The lead can also be an individual, and the role spans valuation plus all core terms at the priced round. Other investors (called "followers") then participate at the terms the lead has negotiated.
In practice, the lead does the structural work of the round. They negotiate the valuation and term sheet with the founders, conduct the deepest diligence, draft (or instruct counsel to draft) the legal documents, coordinate other investors into the round, and usually join the board as a director or observer. Typical lead check sizes scale with stage: roughly $1 million to $3 million of a $4 million...