It means what it says on the tin

You may need to shut down your forums (to prevent people from being online and updating the table), or even turn off mysql and then run a repair on the table.
If you have access to root on your server stop mysql, then repair the table, then restart mysql.
You may need to get your host to do this for you if you are unsure on command line instructions.
As far as I know you should be safe to convert the table to heap.