Traffic and visitor statistics are available for posts you publish on Medium. To view your stats, click your user icon > Stats.
The bar graph represents traffic over the last thirty days. You can go back to the previous thirty days or click on the title of an individual story to see that story's traffic for the last thirty days.
Views vs. Reads
Views are the number of visitors who clicked on a story's page, while Reads tells you how many viewers have read the entire story (an estimate).
Fans represent the number of unique users which clapped for this story.
Clicking on "Referrers" under each post title opens another screen with information breaking about traffic sources for that post.