To explore this, create a simple developer org using the following URL:
https://developer.salesforce.com/signup
Please note that this is for developer organization and so free to experiment. Do not use them for any commercial purposes without talking to Salesforce.
To enable Chatter for Classic use, navigate to Setup | Customize | Chatter Settings.
To enable the new Lightning UI, navigate to Setup | Feature Settings | Chatter Settings. Consider the following screenshot:
Once Chatter is enabled specifically for Lightning experience, you will see the company insights.
The AI ranking brings out your company's most popular content. The ranking of the post is based on latest posts and posts with greater engagement, likes, and comments.