Condition Zero is a fail-safe procedure the Martian Congressional Republic Navy (MCRN) enacts during boarding procedures. It states should any of the three critical areas of the ship be lost to boarders (bridge, Command Information Center (CIC), engineering), the ship is to be detonated (self-destruct) immediately. As these areas are considered vital to ship operation or hold critical strategic value (in the case of the CIC), should these areas fall into enemy hands the Captain (or surviving command officer) should consider the ship lost and initiate self-destruction to prevent the enemy from capturing the ship or otherwise using the ship to enemy advantage.
Excerpt from the book Leviathan Wakes:
Holden gasped out. “Why board you?”
“The command information center,” Alex said. “It’s the holy grail. Codes, deployments, computer cores, the works. Takin’ a flagship’s CIC is a strategist’s wet dream.”
“Cut the chatter,” Kelly said. Holden ignored him.
“That means they’ll blow the core rather than let that happen, right?”
“Yep,” Alex replied. “Standard ops for boarders. Marines hold the bridge, CIC, and engineering. If any of the three is breached, the other two flip the switch. The ship turns into a star for a few seconds.”