Say you have a function/method.
Say you have a number of preconditions that need to be met in that function/method before proceeding deeper into the sub's logic.
Devel::Cover points out which unexercised paths among those preconditions need tests written for them. You write those test, so that the preconditions are 100% tested.
Deeper inside the function.method, Devel::Cover shows you other untested paths.

Looking at these, it quickly becomes apparent that, _if_ the preconditions have been met, and therefore you have progressed to one of these untested code branches, that it is not possible to specify a set of parameters to that function/method that pass the preconditions and progress down the untested path - therefore the untested branch is redundant - it can never be exercised.