A Primary Rate Interface (PRI) is not an esoteric banking term designed to take as much money as possible away from your account. It's a less obscure telecom term that connects your internal PBX to "the outside."
The PRI is the part of the ISDN that is responsible for carrying voice and data from point A to point B. This is essentially the trunk you rent from your telecom provider to connect your internal VoIP lines to the external PSTN. (I wrote a blog about trunks a while back, if you're interested.)
A PRI uses different carrier lines depending on which part of the world you live in. For example, if you live in North America, the PRI uses a T1 line. If you live in Europe, it uses an E1 line. These carriers establish how many external phone lines you can access and how much data you can transmit.
Using the T1 line as an example, there are 24 available channels. These channels are further divided into B channels and D channels. The B channels carry your voice and data, . The D channels carry control and signaling information. A single T1 PRI line has 23 B channels and 1 D channel. If you use more than one PRI line, you can often reduce the total number of D channels. If you lease two PRI lines, you could have 27 B channels and a single D channel, giving you 27 outside telephone lines instead of 26 outside lines.