It's the amount of conventions implemented in a framwork and the number of decision that have been taken.
If, for instance, there are 5 (or more) different ways to submit form data to a controller action (which is the case in ASP.NET MVC), the framework seems to be pretty "un-opinionated" - the decision is up to you!
If, however, the framework enables (either through directly disabling other ways, or by strongly encouraging it) only one way of doing that thing (which is the case with Fubu MVC), you could say that the decision has been taken by the framework, thus making the framework opinionated.
