Refractory material is material with a high condensation temperature (essentially, boiling point), thus turns to solid early as it cools. The opposite is called volatile material. The material in question can be a pure chemical element (refractory element or volatile element), or it can be a compound (refractory compound or volatile compound). A commonly-used line between the two is condensation above/below 1300K.
For example, iron is moderately refractory and lithium is moderately volatile.
This astrophysical usage of the term refractory is perhaps a specialized sense of the word's more general meaning: "heat resistant".