The Secret is that in Chris & Danny's family, their great x1,040 grandfather was a Spartan. He died in war, but his sword was passed down in the family. The sword was made with hand-eye precision, only made on the highest and smoothest rocks in Sparta.
However, there was a catch to the secret. Only first born men in the family knew about the sword from birth, and only they can wield it. Chris & Danny's Uncle Teddy, who was not seen in the series as of yet, was first born. He told his younger brother (Chris & Danny's dad) about the Secret when he was 12, the same age Chris was when Danny told him all about it in the episode.
The sword was only supposed to be used in a crisis, but in the events of the episode, it was a crisis.
Danny used the sword to kill the zombies, and, in the final battle with Mr. Sir when he was a mega zombie, Danny used the sword and stabbed Sir with it. Sir was dead.