From what I understand, it is a term that combines ontology and theology, at least in the case of Heidegger, to describe modern metaphysics. To understand what onto-theology is, I first tried to understand the other two fields of study which are ontology and theology. To put it very basically, the former is the study of being/existence (lowest/basic) while the latter is the study of God (highest/supreme).
However, what do these two seemingly polar fields have in common that they can be combined? And subsequently, what did Heidegger see in modern metaphysics that allowed him to say that "metaphysics is onto-theology"?