PRI basically stands for Primary rate interface. It is basically a standard used in the telecommunications industry. This standard helps in making multiple DSO voice and data transmission possible between a user and a network.
Different countries use different systems for this purpose. T1 system is used by Japan and America. Most of the rest of the world makes use of the E1 system.
The Primary Rate Interface has channels which can be used by large scale organizations. This enables them access to PSTN or Public Switched Telephone Network.