"Storing" is keeping the batteries (fully charged or not) unused for longer period of time: Days, weeks, years... By default, if left charged and not being used, Air's battery will self discharge after 10 days (you can change that in settings) to "storage level", around 60%. If you fully charge the battery, try to charge it shortly before flying, charging one day before is OK. That's why there's auto discharge feature, probably not healthy for the battery to stay charged over days and weeks and not being used.