An open compound is a combination of words so closely associated that they convey the idea of a single concept but are spelled as unconnected words.
The term "open compound" is a misnomer; in fact it's meaningless. "Open" implies separate, but "compound" implies inseparable.
Such expressions are separate words forming a syntactic construction, where the initial item is a modifier of the head. The other kind are morphological compounds composed of two (or occasionally more) smaller bases, e.g. "greenhouse", birdcage", "shortbread", "chewing-gum", "he-man" and so on.
Some are well-established, often lexicalised, while others are concocted on-the-hoof, e.g. "a custard-pie-in-your face sketch", but the distinction is still valid.