Subspace radio is a technique for performing faster than light communication. It is a bit of a misnomer as it often does not use radio signals. It also doesn't use subspace, though the techniques are closely related. Regardless, this form of communication can be one of the most reliable and secure methods of communication over interstellar distances currently known.
The first problem one encounters once subspace travel becomes a thing, is the lag time in communication. Even very short hops in subspace can lead to days or weeks of lag time in communications. Because of this, most species have the same solution to the problem; why not use the same technology they just invented to move themselves vast distances to move data the same way?
Most forms of communication are electromagnetic in nature, meaning they are already moving at the speed of light, in a "massless" form. This has the advantage of making the energy requirements significantly less demanding than moving a ship. But, inlike a ship, it's not really possible to create a pocket that moves with the signal like a subspace pocket moves with the ship that generates it. Instead, space-time between the transmitter and receiver is warped to shorten the distance between them. At this point, any "real time" form of communication can be utilized.
This form of space-time manipulation is known as tunneling. It works best with waves or near-massless particles.
Subspace radio is often used as a point to point communication system. If you know your destination, you can tunnel from your current location to the destination, and then establish a secure form of communication such as a point to point laser system. The resulting communication will be almost impossible to intercept. Most military communications work like this.
Unfortunately, 'nearly impossible to intercept' does not also mean 'nearly impossible to detect'. When using point to point subspace radio (P2PSR) it's possible to detect both the origin and destination of the communication. At the very least that there is tunneling happening is often easily detected. This can lead to detection of otherwise hidden forces simply by their communications. This is why most secure communications use a network of redundant repeater stations. While they can add lag, it also makes it much more difficult to tract down the destination point of a message.
Another technique is what's called a "subspace broadcast". Its possible to tunnel to a nearby location and let the final leg of communication happen at normal lightspeed, over a wide-band radio frequency. This is often more usful for one way rather than two way communication. It is very useful when either the exact destination is unknown, or there's a desire to obfuscate the exact destination.