What does 'explanatory foil' mean?
Ortony's construct of linguistic metaphor (1975) as an explanatory foil.
Article: 'Meaning and visual metaphor' by Hermine Feinstein
The article will discuss Langer's thesis by drawing comparisons with Ortony's construct of linguistic metaphor (whatever that is). The differences between the two will facilitate the discussion.
For simplicity, let's call Langer's thesis "Thing One" and Ortony's construct of linguistic metaphor "Thing Two".
In other words, Thing One is different from Thing Two, and discussing these differences can help us understand Thing One. When we do this, we are using Thing Two as a foil to Thing One. Since we are using this foil to explain Thing One, we can call Thing Two an explanatory foil.