Pro-rata, in the tech ecosystem, refers to a type of investment right that allow an investor to participate in a subsequent round of funding that maintains the percentage of ownership that the investor currently holds in the company.
Pro-rata selection at Investopedia
What are pro-rata investment rights? at FundersClub
Fred Wilson's The Pro-Rata Participation Right at AVC
Mark Suster's What all Entrepreneurs Need to Know About Prorata Rights at Both Sides