Immutable Infrastructure is defined as follows:
"A pattern or strategy for managing services in which infrastructure is divided into "Data" and "Everything else". "Everything else" components are replaced at every deployment, with changes made only by modifying a versioned definition, rather than being updated in-place."
In layman terms, you can reset the phone to factory settings in your smartphone while keeping all the user data intact; and you may keep doing that.
For microservices, when you need to upgrade, you replace the whole service as a fresh one; you do not upgrade or make changes in place in your infrastructure. The data it uses or maintains or builds up remains (mostly) the same. This ensures the application stability and maturity of the microservice.