The blog post by Ian Jennings, part of a series on creating a multiplayer game with JavaScript, explains how to display and update a real-time list of online users in a chatroom using the PubNub JavaScript Presence API. It details the process of setting up a chat interface using Bootstrap's grid system to create a two-column layout, with one column dedicated to displaying online users. The PubNub Presence feature is utilized to track user activity within the chatroom, triggering events when users join or leave and updating the user list accordingly. The blog post emphasizes the use of the unique UUID identifier for each user and the importance of managing the user interface dynamically as users interact with the chatroom. It serves as the third installment in a comprehensive guide covering various aspects of building a multiplayer gaming experience, including matchmaking and creating private chatrooms.