What does it mean when psychologists say that there are biological constraints on classical conditioning?

Looking at the animals (and humans for that matter) the psychologists say that there are biological constraints as both humans and animals can learn the effects of a certain event or a situation. So biologically animals and humans are predisposed to learn how to behave (and also learn skills) in order to survive, that is why they will for instance avoid and have aversion to food that causes them to vomit.