Brandable is a term usually applied to domains that have no obvious use, no traffic, no revenue, and nothing to value them beyond hoping that some day someone will want to buy that particular domain. To me it is the same as buying a lottery ticket. The problem with brandables is it can be very easy to find an alternative for registration fee thus negating the need to pay the owner a premium. I would recommend avoiding these types of domains as a main part of a portfolio.
A traffic domain is just that.. a domain that receives traffic. You park the domain, monitor the traffic, and then a potential buyer can make his decision based on the quality and quantity of the traffic. However it is very unlikely that any new registration (or brandable) will have any natural traffic. You can build traffic to a domain by developing it.