To use the API you will need an API Key or Token that you will pass along your request. You need to sign up to get it, and yes, there is a free plan you can use as long as you want.
If you click on the "clipboard" icon the full API Key string will be copied to your clipboard.
The API is restricted by this API Key. Each subscription plan is limited to a number of API requests per day. If you exceed that limit in a 24 hour period the service will return a 429 HTTP status code. If you need to make more requests you should consider a paid plan with more quota. You can know in real time the quota consumed daily from the Dashboard.
Apility.io expects for the API Key to be included in all API requests made. It can be included as a header or as a query string parameter.
You can learn how to include the API Key in every request in the API documentation.