What is that rapid changing number under Animation Node Tree? I tried to check the doc and couldn't find it, the number under the animation node tree is changing very rapidly like its showing latency. Did I execute something I'm not aware of? I'm new to blender and it's freaking me out to be honest because I don't know what it is doing in the background.
That number shows how long the last execution of your node tree took.

It changes all the time because by default Auto Execution is enabled. If you don't want that you can disable auto execution in the corresponding panel.

It is turned on by default because it makes it easier to get started.