'~만 못하다' means 'worse(not better) than~' so "이야기하는 것이 하지 않는 것만 못할 때도 있어요" directely means "sometimes, to talk is worse(not better) than to do not talk (something)" In this sentence, usually that 'something' means a truth that you don't need to know or don't want to know. You can understand ~것'만' like ~것'보다' 이야기하는 것이/ 하지 않는 것'보다'/ 못하다. I guess It might be easier, isn't it?