If the money in your escrow account is projected to be below your minimum balance at its lowest point in the 12-month period, you have a shortage. This can happen if the taxes or insurance premiums for the previous 12 months were more than expected. Or, if they're estimated to go up in the next 12 months.
Your escrow shortage will be automatically spread over a term of at least 12 months. Your escrow disclosure statement will provide the details of that shortage and any additional information as necessary.