A broken link flag means that one of the links on your hub my no longer be working. This can happen,foe instance, when a page that you are linking to is removed by the website that hosts that page.
As Ohma said, this can also be an error in HubPages!
Open the hub in question and search for the link highlighted in yellow. Click on it - if it goes somewhere it's fine; if not it really is broken and you should remove it or point it somewhere else!
If the link is actually good but being reported as broken, go here:
That will stop the reporting of "broken links" that are not broken!
Hope that helps :D