See previous posts in this series for more about conjugations.
Ser - to be, imperfect subjunctive
Notice that there are two versions, with -ra and -se endings. The two versions are identical as far as I can tell (and I have asked a few Spanish speakers and teachers). Some dictionaries only show the first, which is probably more common, but you will find both used frequently in books, sometimes together e.g. 'fuera lo que fuese' (loosely - 'being whatever it was' or 'whatever it may have been').