What is API?
API meaning Application Programming Interface. For data, this is usually a way provided by the data publisher for programs or apps to read data directly over the web. The app sends the API a query asking for the specific data it needs, e.g. the time of the next bus leaving a particular stop. This allows the app to use the data without downloading the whole dataset, saving bandwidth and ensuring that the data used is the most up-to-date available.
reference: Open Data Handbook – Glossary