What is an API call and what is my limit?

API call limit is set by Salesforce.

The information you require is here:

https://developer.salesforce.com/docs/atlas.en-us.206.0.salesforce_app_limits_cheatsheet.meta/salesforce_app_limits_cheatsheet/salesforce_app_limits_platform_api.htm

Swift digital has an unenforced 1000 calls per 24 hours.