Something that's usable can be successfully used right away for its intended purpose. If your car is usable, it's running just fine.
A usable tent has all of its poles and is more or less waterproof, and a usable pair of shoes fits well and doesn't have holes. Describing something as usable can sometimes be faint praise: "Well, this basketball is usable, but just barely." When information is described as usable, it usually means it's actively available, and not just stored in a computer, for example. This is a relatively new adjective, first appearing around 1840.