In short: Project Fi is emailing those that are in zip codes affected by Hurricane Florence this month, and letting them know that they will be receiving a $20 service credit, to cover any extra data usage that might have happened while there was no power or internet available. This is a bit different than what other carriers typically do for weather events like this, where they offer free data, texts and calls, instead of offering a credit after the fact. Though, it does still have the same impact on those in the area, as they are able to use their phone to check on things.
Background: This isn't the first time that Project Fi has offered service credit following a hurricane or another serious weather event. During Hurricane Harvey last year, Project Fi also offered service credit for those that were in the affected area - most of which was in Texas. It's a good way for users to not worry about using more data when trying to figure out what's going on and if they can return to their home soon. Seeing as there were many people without power for days, this was the easy way to find out when the power was coming back on.
Impact: Hurricane Florence did a ton of damage in the Carolinas, and people are still rebuilding from that Hurricane - and there are more on the way. So it's nice for those in that area to be able to rebuild, and get power back on without worrying about their phone bill going up because they used more data than they normally would have used in a regular month. Not quite the same as getting free data for that period of time, but the $20 service credit is equivalent to two gigabits of data, which can really go a long way.