Many jurisdictions urge people to use OK/HELP signs to facilitate search and rescue after a disaster. If they occupants of a home are all OK after a disaster, then they put the OK side of the sign up in the window so that responders can see that they don’t need to search the premises. On the other hand, if you or another occupant are in need of assistance, you can put up the HELP side of the sign in hopes that a first responder or neighbor can come and provide assistance.
Here is an example sign that you can print out for your own use.
Lower Valley Assembly 