The conditionnel passé (past conditional) describes what would have happened under different circumstances — the French equivalent of "would have done". It's formed with the conditionnel présent of avoir or être plus the past participle: j'aurais fait (I would have done), elle serait venue (she would have come).
It's the standard result clause for a third-type si-clause, describing an unreal past condition: si j'avais su, je serais venu (if I had known, I would have come) — the si-clause itself uses the plus-que-parfait (avais su), while the result uses the conditionnel passé (serais venu). This pairing is fixed and doesn't mix with other tense combinations.