I suppose you do not know what the term 'exception' means...

If i say all my friends are tall with the exception of james, means that james, even if is a friend, is not tall...

So when garv says that a buff is an enhancement that lasts for x rounds, with the exception of barriers, means that barriers, even if are enhancements that last for x rounds, are not a buff...

First read, then if you do not know a term search in a dictionary, then reply.