Funnels are used to track the goal completion rates of a step by step path inside your application. These goals (steps) are defined as custom events in Countly and you don't need any extra/new API calls if you have already been using custom events.

Countly Drill is advanced segmentation feature, available with Enterprise Edition. Drill can be very powerful when it comes to understanding segmented data.

This video provides more information and shows how to use Countly Funnels and Drill.