wher is the Schema.org context?

Hi,

I am having trouble understanding how the json-ld playground uses a context of http://schema.org/ . I thought that at endpoint would be a json-ld document, but no dice. I need the context doc for schema.org for 3 reasons:


1)      Need to include it in an application without network access.

2)      Would like to use it understand how to use json-ld to define a vocabulary.

3)      Drives me nuts not understanding how "playground" knows what a "http://schema.org/Person"  is.

Thank you all,

Robert