An online version is an exact copy of your message intended to display an e-mail in a browser when it cannot be displayed properly in your e-mail client. Depending on your template, the page title in your browser shows the complete url, or the name of your organisation.
For the digital accessibility of your message, it is better to to adjust this title to a more specific description, like "Spotler October newsletter". This makes it easier, for example, to switch tabs by voice.
You can easily set a page title in the General tab of the messages editor. Under General style, you have the space to give a page title for the online version of your message.
Keep in mind that this title is only visible in webbrowsers when viewing your message, and not in the emailclients.
The same is possible for your Spotler Mail+ webpage, but in a different way. For more information on that, read this article: changing the page title of a webpage.