It means that a summary is missing for you hub. I had an A after my hub for a number of months and I kept wondering why. If you go to my account at hub pages and then go to edit on each hub that has an A next to it.
Once you get to edit you need to add a summary or should I say write a summary. While in edit mode look on the right hand side of the hub and you will see a tab that says summary. Once you have written a summary for each of your hubs and then saved that summary all being well that A should disappear. I hope this helps.