What is "cross-buy"?

Cross-buy means you don’t have the buy the game separately on PlayStation 3 and PlayStation 4. Whether you got a code through backing on Kickstarter or bought it in the PlayStation Store, Amplitude is cross-buy, so you can play on both consoles. Have a PlayStation 4 in your living room and a PlayStation 3 in your bedroom? When it's time to retire for the night, you can keep playing from bed without needing to buy the game twice.