A reference document providing a formal description of Org's syntax is
available as a draft on Worg, written and maintained by Nicolas Goaziou. It defines Org's core
internal concepts such as
(greater) elements and
objects. Each part of an Org
file falls into one of the categories above.
To explore the abstract structure of an Org buffer, run this in a buffer:
M-: (org-element-parse-buffer) <RET>
It will output a list containing the buffer's content represented as an abstract structure. The export engine relies on the information stored in this list. Most interactive commands (e.g., for structure editing) also rely on the syntactic meaning of the surrounding context.
You can check syntax in your documents using