A beat is like a cronjob (a periodic task) that you define a URL. Cronbeats will trigger a GET request to the defined URL. You should program that URL to do the task you need to do in your backend application.
In the free version, you can create up to 10 different cron beats.
It is the minimum amount of time between two instances of a cron beat.
Cronbeats is, for now, an MVP (Minimum Viable Product). I'm using it to schedule the tasks of some of my web applications (such as http://mailaday.com/ - to trigger the sending of emails).
When it reaches a mature state where some people is using it without many errors, I will consider to implement the Pro functionalities. If you have been using Cronbeats, and would like to keep using it (in a professional setting), drop me an email (joaojonesventura at gmail).