Broken links are links that don't work anymore. Either the page being linked to is not available anymore, or the link was possibly typed wrong. There are multiple reasons why a link is broken. To fix it go to where you wanted to link to, and use the address bar on top to copy/paste the address to a new link. Delete the old link, or revise it so it directs people where you want the link to go. If it is on one of your hubs just go to edit (on that hub), and replace the link with a new one.