Groups are managed under My Account - Hubs - Groups.
You create groups of your own choosing, put each hub in a group, and then put the hubs in a group in an order that you like. This has four functions:
1) It helps you keep your hubs organized for yourself.
2) If you want your readers to see a set of related articles, or a series (an ordered set), you can arrange it. (Series can include chapters of a longer story or novel, but it also has wider uses.)
3) If you see a set of articles as related, even though they cross into different HubPages topics, you can keep them together. For example, in writing about Go Green, I had hubs that focused on both overpopulation and toxic waste. Two different topics, both in the same group.
4) At the bottom of each Hub, the HubPages website automatically displays the previous and next hub within the group, so readers can follow the series.
There is no direct effect on SEO, but there is a beneficial indirect effect. #4 will increase the odds that readers will stay on your sub-domain. The longer they stay, the more Google sees that readers value your sub-domain, and so your organic SEO on Google goes up.