Basically, Patent Prosecution Highway is a fast track examination of patent applications. It provides a faster process to have a patent application considered by the patent office. The patent application can be prosecuted under PPH only when the applicantion receives a notice from one patent office that at least one of the claims is patentable or allowable.
The applicant then can put in a request to a second patent office where a corresponding application is on file to fast track the examination. This helps the applicant to get through the system faster and more efficiently but with no guarantee that a patent will issue.
Although an application on the Patent Prosecution Highway skips to near the front of the queue, in theory they are still examined as rigouously as any other application. In practice they have a higher allowance rate.
As for whether or not the claims are patentable, depends on what the allowable subject matter is according to the patent law of any particular country.
You can get more information from the USPTO website about PPH in the US and for other countries provided in the link below: