Quite simply, earned media refers to the exposure you've earned through word-of-mouth. Whether that's content you've distributed on other websites (e.g., guest posts), PR work you've been doing, or the customer experience you've delivered, earned media is the recognition you receive as a result. You can earn media by getting press mentions, positive reviews, and by other people sharing your content on social media, for instance.
Because it's such a popular communication channel, it has wide-ranging integrations with plenty of other tools. You can tack on a seemingly unlimited amount of integrations and Slack applications to make the tool even more powerful -- for instance, many teams will pipe in A/B test results, analytics notifications, new customer or transaction notifications, or even sales or customer support bots. While undeniably powerful, Slack is easy to start using immediately.