What is the y-axis of this Oracle ASH Viewer?

I am investigating different monitoring software for my database, and I stumbled upon this open source ASH Viewer. I noticed in the main graph on the y-axis for resource usage, it's labeled as "Active Sessions" with an axis of varying integers Link: https://github.com/akardapolov/ASH-Viewer#how-it-works I don't understand what "Active Sessions" is supposed to mean. Is this the CPU % utilization of the database?