Follow-on investors fill out the round after the lead has set terms. They contribute capital, sometimes specialized value-add (sector expertise, customer introductions, hiring), and typically don't negotiate terms. A typical Series A might have one lead and three to five follow-ons.
Follow-on dynamics matter for round closing speed and quality. A round with a credible lead and engaged follow-ons closes quickly; one with a lead but no follow-on interest can stall as the lead becomes uncertain about the syndicate.