Pro rata rights give an existing investor the right, but not the obligation, to invest in a future round to maintain their current ownership percentage, rather than being diluted down like anyone who doesn't participate.
Without pro rata rights, an investor's ownership percentage shrinks with every new round, the same as anyone else's. With them, that investor can write a check in the next round sized to keep their percentage roughly where it was, if they choose to exercise the right.
Early investors who believe strongly in a company often value pro rata rights as much as the initial investment terms, since it's how they protect their ability to keep backing a winner rather than getting diluted out of a deal they were early to. Worth negotiating clearly on both sides, since founders also need to know how much room a future round actually has for new investors.
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