It is a written, formal way of expressing something like "even if". A similar construction exists in English, with subjunctive and inversion, and similar meaning, e.g.:
Les fantômes vivent rarement dans les bois, et encore moins dans les maisons, fussent-elles à l'abandon.
Ghosts live rarely in the woods, and even less in homes, be they abandoned.
Note the special form of subjunctive used in these context for first-person cases:
Aussi, fussé-je seul contre tous, je défendrai Voltaire jusqu'à mon dernier soupir.
Therefore, be I alone against all, I will defend Voltaire until my last breath.
Even though grammatically any verb could be used this way, in practice I don't think there are many other verbs besides être (fussé-je, fusses-tu, fût-il, fussions-nous, fussiez-vous, fussent-ils) that are actually used with this construction. I've heard avoir and devoir, e.g.:
Eussé-je dit mille fois qu'il fallait m'écouter, cela n'eût servi à rien.
Had I said a thousand times that I should be listened to, it wouldn't have been any help.
With devoir:
Dussé-je en périr, je te serai fidèle jusqu'au bout.
Even if I should perish from it, I will be faithful to you until the end.