Your Plan details page defines the total amount of included data transfer, operations and storage available.
Data transfer is the amount of data in and out of the service that is transferred.
Operations is the number of Login, Get and Put operations executed.
Storage is the amount of data storage used (No storage is used for streaming transfer services like Simple FTP Service / SFS).
If you reached your limits and are not on a paid plan the service will stop working immediately - However, if you are on a paid plan, additional pricing as listed on the pricing page comes into effect.
This means that at the end of your billing cycle an additional amount of usage will be debited on your next bill to cover the cost of your usage for the last period of the last billing cycle.