Here are details from USA Today.
What this means is that you can leave a day early (or otherwise adjust your plans) without additional charge. If you are headed to the storm area (see below), it would be a good idea to leave tomorrow (no precipitation) or as early as possible Wednesday before storm conditions worsen.
Here is the step-by-step of how to handle it. Remember: If your hub (a place where you change planes) is affected by the storm, it can cause even more problems than if your destination or origin airport has adverse weather.