Compuserve web mail is an email service provider from the USA, they were the first company to render commercial online service. It has various features like forums, chat systems, software downloads or even various other services. If you want to understand easily then know that this is much older than recent social media like facebook, whatsapp, twitter etc.
Also read:
https://saramax092.wixsite.com/expertsbadge/post/does-compuserve-email-still-exist