The phrase "to smash the berries" means to crush or break the small juicy fruits called berries into smaller pieces using force, usually with the intention to use them for cooking or eating. Full definition
Remove from heat and use a fork to gently smash the berries. (pastryaffair.com)
Add in the remaining ingredients and simmer for 5 minutes, stirring occasionally and smashing the berries with the back of a spoon. (withfoodandlove.com)