No, they are taking an unfertilized egg from one mother, removing its ooplasm---the material surrounding the nucleus, where the chromosomes are---and injecting that ooplasm into the fertilized egg. I'm not sure whether it is a full replacement of the fertilized egg's ooplasm or whether it is simply an addition to it.
The reason they would do this is to counter genetic defects in the embryo's mitochondrial DNA. This is DNA that is passed only from the mother and is not in the 46 chromosomes themselves. If that DNA is defective it can lead to certain disorders like some muscular dystrophy-like symptoms.
Again, not to defend the practice as ethical necessarily, but it is worth pointing out that this is NOT done simply to create a designer baby but rather to correct known and significant defects. It likely wouldn't pass the ethical test for anyone who believes life should be protected from conception, however, because it puts the egg at risk not commensurate with the severity of the disorder.