IMO stress relieving is a catchall used interchangeable to describe resolving a number of issues during/after a build.

Different techniques do different things better or worse. IME the two main things dealt with are allowing spokes to untwist, and setting the bend of the elbows.

Unresolved spoke twist can cause a new wheel to go slightly out of true early on, which is why many builders used to suggest that clients ride a while then have the wheel checked and/or tuned up. This ride/tuneup step can be resolved before the wheel is sent out, and most better builders today do so.

IME- setting the bend is probably the more important step, and may be the one thing that most separates a good hand built wheel from a production wheel. Steel has spring properties which cause something called "springback" or a return back from the bent to position. We see this all the time when doing something like straightening frames where we need to over-bend so the frame springs back to where we want it.

When you tighten a wheel the elbows will bend to the final angle imposed by the tension. There's no mechanism to over-bend and allow for spring- back, so the elbow stays in the loaded (stressed) condition. Setting the bend by over-bending slightly allows the elbow to relax back to where it would end up when set purely by tension.

Eery builder I know has his own favorite technique(s) for "stress relieving", and might call the process by a different name, but it boils down to the same thing.