What is a REST API?
A REST API is a way to query a system with a simple HTTP request.
The easiest way to do that is to use cURL (on linux). For example, the command below will return the statistics of the main public instance:
curl -X 'GET' 'https://lookyloo.circl.lu/json/stats' -H 'accept: application/json'
The documentation of the REST API is automatically generated and avaliable over the lookyloo web interface. If you want to run queries against the public instance, you can use this documentation.