Overage is when you've used more data than what's allocated to your plan, say that you got one of those Hedgehog VPS plans, that means you have 1000GB data transfer limit per month. You can see both the limit and current usage in your dashboard: Dashboard -> My servers then click on the server you'd like to view and select view information.
Then you'll see a value like 9 / 1000 where 9 is your current usage in GB and 1000 your servers total limit. They also only count the biggest value out of incoming vs outgoing traffic so if you have 100GB incoming and 200GB outgoing then your current usage will be 200GB. These limits are pretty generous tho and I really don't think you need to worry about overage. 1TB data in a month is a lot for a server with just 1CPU and 1GB RAM, bigger servers has even higher limits so you might wanna check one of those out as well.