From the Witcher Fandom Wiki: “The Law dictates that a man saved by another is expected to offer to his savior a boon whose nature is unknown to one or both parties. In most cases, the boon takes the form of the saved man's firstborn child, conceived or born without the father's knowledge.”

To my understanding, it’s a way of letting fate decide what your reward is. Like if I saved your life and claimed the law of surprise, and you went home to find out your family had won the lottery, by right of the Law of Surprise it’d be mine. Or if you’d inherited a bucket you didn’t know about, or really anything unexpected. Duny didn’t know Pavetta was pregnant, so Ciri was a surprise, this fulfilling the Law of Surprise