"Alas, Babylon" is the code phrase that Randy and Mark Bragg come up with for "the end is nigh." Mark's alert message is sent by telegraph and passes through several hands before reaching Randy. No way Mark could have sent "the Russians are planning on attacking" or something similar.
Today, we don't have to deal with telegraphs, but electronic communication is subject to many of the same weaknesses--hacking, eavesdropping, and so on. If you're in public, telling the family something like "I think that guy has a gun, let's get out of here" may some unwanted attention. In the event that you are captured, kidnapped or otherwise compromised, and the enemy is attempting to use you to get to friends, family or whoever, you can use a simple, innocuous word to warn them, even while the bad guy listens in.
Code words are a very good idea--they allow you to communicate something important without raising alarm or suspicion, and do it very quickly. The code word/words should be something that you wouldn't normally say--something that will be obviously out of place to the person you're communicating with, but something that a stranger, enemy, etc. wouldn't recognize. Discuss it and plan out the reaction with the intended recipient.
For example, say you want to establish a code word to tell your wife to bug out. You plan out the response-- she grabs the bug out bags, loads up the family vehicle, pulls the kids out of school and heads to her sister's place up north (or a motel, or wherever). You'll meet her later. Then come up with the code phrase--"Mr. Schumer says hello." Discuss the importance and seriousness, and then leave it at that. Now, if something happens, you can get you family moving to safety with a simple phrase.
If you have older kids with cell phones, set up code phrases with them--if they're in trouble, they can let you know without drawing attention. If you have younger kids, you can have a set code phrase given to a friend/family member if you need to change plans and have them picked up. Similar to code phrases are challenge/response phrases--commonly seen in spy movies as something like "the crows flies at midnight." They can be used to identify a stranger as a friendly, or can be used to query someone if you're worried they're in a bad situation.
Code phrases. Pretty powerful, and zero cost to set up.
