Welcome @Brenda_Blaze.
To simply put, you have a Third Party Software Application that is connected to your Keap Application. You have software that either gets or pushes data into your Keap Account. This software could be in the form of Opt In Forms, Subscriber Forms, Order Forms, etc. The challenge is finding the software that is using the Legacy API Key.
I have written a guide via the link below to help people identify what integrations they may be using, although it is technical given the nature of things.
https://integration.keap.com/t/legacy-api-key-identification-guide/93019
In your Keap Application 3,723 API Calls were made using the Legacy API Key. That means on a daily average: 3723 / 30 = 124 API Calls are being made. Usually it would be some sort of Opt In Form that would be in operation. Do you have 100+ Contacts being added into your application on a daily basis?
You need to get the API Call information from Keap Support, then take a look at the guide which will explain what the API Calls mean. If you are unsure, just post the information here.
Hope that was clear.