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".