The contractual right (not the obligation) for an investor to buy enough of a future round to keep their current ownership percentage intact.
If a seed investor owns 5% at the time of investment, they want the right to maintain that 5% through Series A, B, C. Without pro rata, their early bet gets diluted to a fraction of a percent by the time it actually matters. For Sequoia, Benchmark, etc., signing a seed deal WITHOUT pro rata is essentially unheard of.