They're links to other webpages. If you highlight a word then select the 'link' tab, then you can make that word a link to either of one of your related hubs, or someone else's hub, or in fact any other webpage on the internet.
Ideally if you use related words, then it will help the page that you're linking to with google searches. So lets say you've written a hub about shark fishing, you have a related hub about crab fishing for example and you want to make a link from the shark hub to the crab hub. This is how you'd do it.
Open the crab fishing hub, highlight the url (the thing that starts with http:// in the address bar) and copy that with Ctrl +C.
On your shark fishing hub, somewhere you'd write the words crab fishing, then you would highlight the words, select the link button, then make those into a link by pasting the url from earlier with Ctrl +C into the correct box.
This is a backlink to the crabfishing hub from the shark fishing hub. When people read the hub, they'll see the words crab fishing highlighted in blue. When google crawls the page, it will see that the page that you've linked with this link is about crab fishing. A link like this that uses the keywords relevant to the site in question is called an anchor link.
The idea is that if this is done enough times, when people type crab fishing into google, then the page that was linked to should show up. The more of those links you get, the higher the page ranks in google.
I hope that helps!