All bandwidth limits across all plans are "soft" limits. This means your site will continue to operate even if you exceeded your limits but will be charged per usage afterward.
For example, you have a site with Pro plan which gives you 500 GB of included bandwidth. Once the total traffic of this site exceeded 500 GB within the billing cycle, $0.25* will be charged per 1 GB used.
* The price for every plan and every bandwidth tier is different, please see the pricing page for more details.