"Opinionated" means the software has made very deliberate design tradeoffs and is very inflexible/resistant to changing that. This is done to optimize certain objectives like software scope or increasing productivity by enforcing certain methods.

So in this case they cited "no client-side routing wiz-bang" -- that's one of their opinions and won't be introducing any support for that sort of thing.

The classic example is Python's motto "There's Only One Way To Do It", which was in a response to Perl's "There's more than one way to do it."

Another example is Ruby On Rails...it has a strong opinion on how the application should be structured -- must be MVC, most of your code should be in a model, convention over configuration, etc.

Generally, rather than trying to be a generic framework/tool/system allowing all approaches and preferences and practices, it is optimized for / selects a certain subset toward a (hopefully) stated goal and subset.

It is "opinionated" in the sense that it has an "opinion" on what's the right way to accomplish a certain outcome, and thus it preselects/premakes some choices for you.

The implicit tradeoff is that it may not be as good at general problems / all approaches, but may be better or easier or both for specific goals / approaches.

It means that the framework makes certain choices and trade-offs, and doesn't allow the developer using it a choice in those matters.