a brandable name is one that other companies dont have trademarks on or been doing business with these names. It can also be a generic product to some degree. It is a name that a company can build a brand around in fact you have some in your signature
A big mistake for new domainers is they have a huge portfolio of brandable names that anyone could make up themselves like google did or many others. Best thing is to concentrate on traffic names and only keep a few brandables, unless they represent generic products. Just my opinion here of coarse