Pro rata rights give an investor the option, not the obligation, to invest more in future rounds in order to keep their ownership percentage from shrinking as new shares are issued. It is a quiet term with outsized long-term consequences.
Every new financing issues new shares, which dilutes existing holders. An investor who owned a given percentage after one round will own less after the next, unless they put in enough new money to maintain their share. Pro rata rights reserve them the ability to do exactly that, buying their proportional slice of the new round.
For investors, pro rata is how they double down on winners; the right to keep buying into a company that is working is worth a great deal. For founders, granting broad pro rata rights can crowd out room for new investors in later rounds, so the term is a negotiation, not a formality. Super pro rata rights, which let an investor buy more than their share, are more aggressive still.
A right is only as good as the capital behind it. Investors who cannot fund their pro rata get diluted anyway, and founders should understand which of their investors can actually follow on. The right shapes who controls the cap table over time. This is general information, not investment advice.
The option to not get diluted away.