The Airtable REST API is a way that the world outside of an Airtable base can interact with an Airtable base. Some major difference between using the REST API and scripting or custom apps:
- the code typically resides outside of Airtable, such as on a web server
- the code can be written in a language other than JavaScript.
To learn specifically about the Airtable REST API, look at the documentation: https://api.airtable.com.