A category (as distinguished from a metacategory) will mean any interpretation of the category axioms within set theory. Here are the details.
Sect-2,page-10, Category Theory for the Working Mathematician.
Is there an exact definition of interpretation in Mathematics? Also is there a general meta theory about interpretable structures?
Per nLab,
A category in general doesn't have to be representable by a set (a "set" depending on the foundation of mathematics you are using), in fact such categories are generally called "small". You can see in this question some examples of "large" categories.
I don't believe interpretation has any specific meaning in mathematics, it's just the word Mac Lane uses here to describe the idea of restricting the axioms only to sets.