A Blackfire recommendation is a best practice that we advise to improve your application's performance.
Anytime you profile an application, Blackfire can detect common performance bottlenecks and configuration issues. Granting you the time and information to solve them, most of the time significantly improving application performance.
Technically, a recommendation is a built-in performance test, that shows up only when relevant. It depends for instance on the detected framework and whether you are in a development environment or not.
Recommendations are displayed in a dedicated tab on a profile view and in build reports. Any recommendation can be overridden and imported within your own tests. This could allow you to have threshold values that align with your application's current state.