Mirror life is when you have a copy of something that looks exactly like the original thing, but it's not a real thing. For example, when you stand in front of a mirror, you see a reflection of yourself that looks just like you, but it's not actually another person.
In the same way, mirror life is used to create copies of living things such as cells or even entire organisms. Scientists use this technique to understand how these organisms work, and to try and find new ways to fight diseases.
It's like making a photocopy of a picture. You can make more than one copy of the picture, but they will all look exactly the same as the original. With mirror life, we can make exact copies of living things, but they won't be the real thing.