A that is a Javascript object assembled according to the philosophy of that-ism. Rather than using Javascript's dangerous this, and prototypal and constructor idioms, a that is a simple direct assemblage of functions and data, put together in a closure scope which is used to house a reference to itself. Since this hidden that cannot be rebound, this kind of object is safe from tampering. The name that-ism is taken from its first successful presentation by Douglas Crockford, by contrast with this. Every top-level Fluid Component is a that, as well as many other objects around the framework.
More details and references on that-ism are in the glossary at Framework Glossary#that.