I think you are probably referring to the ability of some lizards to drop their tails in self-defense against a predator and to later heal and regrow (regenerate) the tail?

This process isn't totally understood yet, but scientists have been able to identify a few hundred specific genes that are activated in the lizard's tail for regeneration. Interestingly, humans also possess most of those same genes, which means that with better understanding of the lizard's regeneration process, we might be able to advance human medicine for regeneration of limbs.

The genes control the behavior of the cells in the lizard's tail. In an animal's body, cells originate as stem cells, which can turn into any kind of specific cell depending on where in the body they are located during development. We say that the stem cells "differentiate" to become specific cells such as skin cells or muscle cells, or light-sensitive cells in the eyes or many other options. The genes in the lizard direct the cells to "dedifferentiate," or in other words, to be made back into something like stem cells which can be reprogrammed to build new tissues as needed to construct a new tail.

Reptiles heal the same way that mammals do. Some reptiles are able to regrow lost tails, but this is just something that their cells can do and mammals' can't.