The best way to explain how the silver sword is used as a symbol in The Silver Sword by Ian Serraillier is to direct your attention to what Jan tells Ruth on the night that Ruth realizes Edek might die as he lay beside them wracked with a cough, pain and exhaustion. First, Jan asks Ruth if he may have Edek's shoes when he dies. Ruth forces herself to reply calmly, "He's not going to die." It is then that Jan makes a clear statement of the silver sword's use as a symbol.
Jan explains that the children's lives and good fortunes during their quest to reunite with Edek's and Ruth's father and their journey to the safety of Switzerland depends upon the possession of the silver sword. He explains that this is so because their father gave it to him, entrusting him with it, thereby building a "lifeline" between children--including Jan--and father. Therefore, the silver sword is a symbol of survival, of a lifeline through troubled times. It further symbolizes the trust that the father placed in Jan. In a larger picture, the silver sword also symbolizes the same kind of hope and trust for all the similarly suffering people the children encounter. Here's what Jan tells Ruth:
Once more out of the stillness a voice called her name. This time it was Jan.
"Ruth, may I have Edek's shoes when he dies?" he said.
"He's not going to die," said Ruth, forcing herself to speak calmly.
"He will if I don't have my sword," said Jan. "And we'll never find your father either. He gave me the sword and it's our guide and lifeline. We can't do without it."
He spoke with such certainty that she almost believed him. It was true that, while they had the sword, fortune had been kind to them.
The ideas of survival, a lifeline, and trust are major themes in the story and are thus represented by the silver sword.