In languages with dynamic properties on objects, it’s a data structure that keeps track of which properties are present, and how to access them.

Short example In JavaScript: Var a = {}; a.b = 15;

The hidden class object a maps the property b to where its stored, may include type information, etc.

The optimization is usually called shapes outside of JS.

The idea is that shapes (we represent objects like C like structs and not hash maps) are used for relatively non-dynamic objects and then we keep the shape metadata on functions themselves (inline caches) which means property access becomes just a byte offset.

See https://v8.dev/blog/fast-properties