Python DB-API is a standard to which different Python database adapters ought to conform. It is a specification that states a set of required objects and database access processes to provide uniform behaviour across different database adapters.
It is a good thing as it allows for the database adapters to be similar enough to be learned fast and switched to and from swiftly. This enables rapid prototyping and testing and allows the developer to choose the best database adapter and the underlying database system quickly.