This rule states that no parameters should be a of a primitive type (e.g. int, double) nor a string. Instead new objects should be created to represent what the parameters are. Is the integer really representing money or a coordinate. This small classes also creates great places to put methods that manipulate whatever the primitive type represents.
When I first tried the object calisthenics this was one of the harder rules actually. Not by itself but because of the ninth rule; don’t use getters or setters. Together it really makes you have to think about how to represent things in your code.
By itself I think this rule is one rule that easily translates to your real code. Making sure you’re passing objects to your methods which actually are what they represent rather than a primitive type that represent something is generally a good thing if you some day have to change the representation. E.g. money should be a double rather than an integer.