It's a standalone application that will push all of the messages from the channel that your Slack bot is in to Apache Kafka. They're pushed as JSON.
If you don't have a bot set up in your channel then you'll need to do that first. There are good instructions available to show you how to do this. Then you'll need to invite the bot into channels that you're interested in receiving messages from.
When you set up the Slack bot, you'll be given an API token. This is the token that you'll need to provide to this program for the Slack connection to work. Note it down somewhere!
Copy the application_sample.yml file to a new file called application.yml in the same folder. That's where you'll put your Slack bot's API token.
You may already have Kafka running on your machine or on a remote machine. If so, then good for you! Provide the list of VIP brokers to the application.yml file along with the topic that you'll want messages pushed to.
Simply do gradle run.