Android is a Linux OS. On Linux there are two access modes: as "root", ie as admin, or as "user". So a rooted Android phone means that you can access it with root privilege, ie admin rights. Now some features in Avast Mobile Security require that you have root access to your phone, which is not the case by default. Getting root access then, if you really want it, requires that you run some software first that will "unlock" your OS.